Hard disk drive

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Hard disk drive
Video of modern HDD operation (cover removed)
Date invented 24 December 1954[note 1]
Invented by IBM team led by Rey Johnson
A disassembled and labeled 1997
HDD. All major components were
placed on a mirror, which created the
symmetrical reflections
Overview of how an HDD functions
Hard disk drive
From Wikipedia, the free encyclopedia
A hard disk drive (HDD)[note 2] is a data storage device used
for storing and retrieving digital information using rapidly
rotating disks (platters) coated with magnetic material. An HDD
retains its data even when powered off. Data is read in a
random-access manner, meaning individual blocks of data can
be stored or retrieved in any order rather than sequentially. An
HDD consists of one or more rigid ("hard") rapidly rotating
disks (platters) with magnetic heads arranged on a moving
actuator arm to read and write data to the surfaces.
Introduced by IBM in 1956,[2] HDDs became the dominant
secondary storage device for general purpose computers by the
early 1960s. Continuously improved, HDDs have maintained
this position into the modern era of servers and personal
computers. More than 200 companies have produced HDD
units, though most current units are manufactured by Seagate,
Toshiba and Western Digital. Worldwide revenues for HDDs shipments are
expected to reach $33 billion in 2013, a decrease of about 12% from
$37.8 billion in 2012.
The primary characteristics of an HDD are its capacity and performance.
Capacity is specified in unit prefixes corresponding to powers of 1000: a 1-
terabyte (TB) drive has a capacity of 1,000 gigabytes (GB; where 1
gigabyte = 1 billion bytes). Typically, some of an HDD's capacity is
unavailable to the user because it is used by the file system and the
computer operating system, and possibly inbuilt redundancy for error
correction and recovery. Performance is specified by the time to move the
heads to a file (Average Access Time) plus the time it takes for the file to
move under its head (average latency, a function of the physical rotational
speed in revolutions per minute) and the speed at which the file is
transmitted (data rate).
The two most common form factors for modern HDDs are 3.5-inch in
desktop computers and 2.5-inch in laptops. HDDs are connected to
systems by standard interface cables such as SATA (Serial ATA), USB or
SAS (Serial attached SCSI) cables.
As of 2012, the primary competing technology for secondary storage is
flash memory in the form of solid-state drives (SSDs). HDDs are expected
to remain the dominant medium for secondary storage due to predicted continuing advantages in recording capacity
and price per unit of storage;[3][4] but SSDs are replacing HDDs where speed, power consumption and durability are
more important considerations than price and capacity.[5][6]

Contents
1 History
2 Technology
2.1 Magnetic recording
2.2 Components
2.3 Error handling
2.4 Future development
3 Capacity
3.1 Calculation
3.2 Redundancy
3.3 File system use
3.4 Units
4 Form factors
5 Performance characteristics
5.1 Time to access data
5.2 Seek time
5.3 Latency
5.4 Data transfer rate
5.5 Other considerations
6 Access and interfaces
7 Integrity and failure
8 External removable drives
9 Market segments
10 Manufacturers and sales
11 Icons
12 See also
13 Notes
14 References
15 Further reading
16 External links
History
Main article: History of hard disk drives
HDDs were introduced in 1956 as data storage for an IBM real-time transaction processing computer[2] and were
developed for use with general purpose mainframe and minicomputers. The first IBM drive, the 350 RAMAC, was
approximately the size of two refrigerators and stored 5 million 6-bit characters (the equivalent of 3.75 million 8-bit
bytes) on a stack of 50 disks.

In 1961 IBM introduced the model 1311 disk drive, which was about the size of a washing machine and stored two
million characters on a removable disk pack. Users could buy additional packs and interchange them as needed,
much like reels of magnetic tape. Later models of removable pack drives, from IBM and others, became the norm in
most computer installations and reached capacities of 300 megabytes by the early 1980s. Non-removable HDDs
were called fixed disk drives.
Some high performance HDDs were manufactured with one head per track, e.g., IBM 2305 so that no time was lost
physically moving the heads to a track.[7] Known as Fixed-Head or Head-Per-Track disk drives they were very
expensive and are no longer in production.[8]
In 1973, IBM introduced a new type of HDD codenamed "Winchester". Its primary distinguishing feature was that
the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down.
Instead, the heads were allowed to "land" on a special area of the disk surface upon spin-down, "taking off" again
when the disk was later powered on. This greatly reduced the cost of the head actuator mechanism, but precluded
removing just the disks from the drive as was done with the disk packs of the day. Instead, the first models of
"Winchester technology" drives featured a removable disk module, which included both the disk pack and the head
assembly, leaving the actuator motor in the drive upon removal. Later "Winchester" drives abandoned the removable
media concept and returned to non-removable platters.
Like the first removable pack drive, the first "Winchester" drives used platters 14 inches (360 mm) in diameter. A few
years later, designers were exploring the possibility that physically smaller platters might offer advantages. Drives with
non-removable eight-inch platters appeared, and then drives that used a 5 1⁄4 in (130 mm) form factor (a mounting
width equivalent to that used by contemporary floppy disk drives). The latter were primarily intended for the then-
fledgling personal computer (PC) market.
As the 1980s began, HDDs were a rare and very expensive additional feature on PCs; however by the late 1980s,
their cost had been reduced to the point where they were standard on all but the cheapest PC.
Most HDDs in the early 1980s were sold to PC end users as an external, add-on subsystem. The subsystem was not
sold under the drive manufacturer's name but under the subsystem manufacturer's name such as Corvus Systems and
Tallgrass Technologies, or under the PC system manufacturer's name such as the Apple ProFile. The IBM PC/XT in
1983 included an internal 10MB HDD, and soon thereafter internal HDDs proliferated on personal computers.
External HDDs remained popular for much longer on the Apple Macintosh. Every Mac made between 1986 and
1998 has a SCSI port on the back, making external expansion easy; also, "toaster" Compact Macs did not have
easily accessible HDD bays (or, in the case of the Mac Plus, any hard drive bay at all), so on those models, external
SCSI disks were the only reasonable option.
Driven by areal density doubling every two to four years since their invention (an observation known as Kryder's law,
similar to Moore's Law), HDDs have continuously improved their characteristics; a few highlights include:
Capacity per HDD increasing from 3.75 megabytes[2] to 4 terabytes or more, more than a million times larger.
Physical volume of HDD decreasing from 68 cubic feet (1.9 m3)[2] (comparable to a large side-by-side
refrigerator), to less than 20 millilitres (0.70 imp fl oz; 0.68 US fl oz),[9] a 100,000-to-1 decrease.
Weight decreasing from 2,000 pounds (910 kg)[2] to 48 grams (1.7 oz),[9] a 20,000-to-1 decrease.
Price decreasing from about US$15,000 per megabyte[10] to less than $0.00006 per megabyte ($90/1.5
terabyte), a greater than 250-million-to-1 decrease.[11]
Average Access Time decreasing from over 100 milliseconds to a few milliseconds, a greater than 40-to-1

Magnetic cross section & frequency
modulation encoded binary data
Diagram labeling the major
components of a computer HDD
Recording of single magnetisations of
bits on a 200MB HDD-platter
(recording made visible using CMOS-
MagView).[17]
improvement.
Market application expanding from mainframe computers of the late 1950s to most mass storage applications
including computers and consumer applications such as storage of entertainment content.
Technology
Magnetic recording
See also: Magnetic storage
An HDD records data by magnetizing a thin film of ferromagnetic
material[note 3] on a disk. Sequential changes in the direction of
magnetization represent binary data bits. The data is read from the disk by
detecting the transitions in magnetization. User data is encoded using an
encoding scheme, such as run-length limited encoding,[note 4] which
determines how the data is represented by the magnetic transitions.
A typical HDD design consists of a spindle that holds flat circular disks, also called platters, which hold the recorded
data. The platters are made from a non-magnetic material, usually aluminium alloy, glass, or ceramic, and are coated
with a shallow layer of magnetic material typically 10–20 nm in depth, with an outer layer of carbon for
protection.[13][14][15] For reference, a standard piece of copy paper is 0.07–0.18 millimetre (70,000–
180,000 nm).[16]
The platters in contemporary
HDDs are spun at speeds varying
from 4,200 rpm in energy-
efficient portable devices, to
15,000 rpm for high performance
servers.[18] The first HDDs spun
at 1,200 rpm[2] and, for many
years, 3,600 rpm was the
norm.[19] Today, the platters in
most consumer HDDs spin in the
range of 5,400 rpm to 7,200
rpm.
Information is written to and read from a platter as it rotates past devices
called read-and-write heads that operate very close (often tens of
nanometers) over the magnetic surface. The read-and-write head is used to
detect and modify the magnetization of the material immediately under it.
In modern drives there is one head for each magnetic platter surface on the spindle, mounted on a common arm. An
actuator arm (or access arm) moves the heads on an arc (roughly radially) across the platters as they spin, allowing
each head to access almost the entire surface of the platter as it spins. The arm is moved using a voice coil actuator or
in some older designs a stepper motor. Early hard disk drives wrote data at some constant bits per second, resulting
in all tracks having the same amount of data per track but modern drives (since the 1990s) use zone bit recording—
increasing the write speed from inner to outer zone and thereby storing more data per track in the outer zones.

Longitudinal recording (standard) &
perpendicular recording diagram
HDD with disks and motor hub
removed exposing copper colored
stator coils surrounding a bearing in
the center of the spindle motor.
Orange stripe along the side of the
arm is thin printed-circuit cable,
spindle bearing is in the center and the
actuator is in the upper left
Head stack with an actuator coil on
the left and read/write heads on the
right
In modern drives, the small size of the magnetic regions creates the danger that their magnetic state might be lost
because of thermal effects. To counter this, the platters are coated with two parallel magnetic layers, separated by a
3-atom layer of the non-magnetic element ruthenium, and the two layers are magnetized in opposite orientation, thus
reinforcing each other.[20] Another technology used to overcome thermal effects to allow greater recording densities is
perpendicular recording, first shipped in 2005,[21] and as of 2007 the technology was used in many HDDs.[22][23][24]
Components
A typical HDD has two electric
motors; a spindle motor that
spins the disks and an actuator
(motor) that positions the
read/write head assembly across
the spinning disks. The disk
motor has an external rotor
attached to the disks; the stator
windings are fixed in place.
Opposite the actuator at the end
of the head support arm is the
read-write head; thin printed-
circuit cables connect the read-
write heads to amplifier electronics mounted at the pivot of the actuator.
The head support arm is very light, but also stiff; in modern drives,
acceleration at the head reaches 550 g.
The actuator is a permanent
magnet and moving coil motor
that swings the heads to the desired position. A metal plate supports a
squat neodymium-iron-boron (NIB) high-flux magnet. Beneath this plate is
the moving coil, often referred to as the voice coil by analogy to the coil in
loudspeakers, which is attached to the actuator hub, and beneath that is a
second NIB magnet, mounted on the bottom plate of the motor (some
drives only have one magnet).
The voice coil itself is shaped rather like an arrowhead, and made of doubly
coated copper magnet wire. The inner layer is insulation, and the outer is
thermoplastic, which bonds the coil together after it is wound on a form,
making it self-supporting. The portions of the coil along the two sides of the
arrowhead (which point to the actuator bearing center) interact with the
magnetic field, developing a tangential force that rotates the actuator. Current flowing radially outward along one side
of the arrowhead and radially inward on the other produces the tangential force. If the magnetic field were uniform,
each side would generate opposing forces that would cancel each other out. Therefore the surface of the magnet is
half N pole, half S pole, with the radial dividing line in the middle, causing the two sides of the coil to see opposite
magnetic fields and produce forces that add instead of canceling. Currents along the top and bottom of the coil
produce radial forces that do not rotate the head.

The HDD's electronics control the movement of the actuator and the rotation of the disk, and perform reads and
writes on demand from the disk controller. Feedback of the drive electronics is accomplished by means of special
segments of the disk dedicated to servo feedback. These are either complete concentric circles (in the case of
dedicated servo technology), or segments interspersed with real data (in the case of embedded servo technology).
The servo feedback optimizes the signal to noise ratio of the GMR sensors by adjusting the voice-coil of the actuated
arm. The spinning of the disk also uses a servo motor. Modern disk firmware is capable of scheduling reads and
writes efficiently on the platter surfaces and remapping sectors of the media which have failed.
Error handling
Modern drives make extensive use of error correction codes (ECCs), particularly Reed–Solomon error correction.
These techniques store extra bits, determined by mathematical formulas, for each block of data; the extra bits allow
many errors to be corrected invisibly. The extra bits themselves take up space on the HDD, but allow higher
recording densities to be employed without causing uncorrectable errors, resulting in much larger storage capacity.[25]
In the newest drives of 2009, low-density parity-check codes (LDPC) were supplanting Reed-Solomon; LDPC
codes enable performance close to the Shannon Limit and thus provide the highest storage density available.[26]
Typical HDDs attempt to "remap" the data in a physical sector that is failing to a spare physical sector—hopefully
while the errors in the bad sector are still few enough that the ECC can recover the data without loss. The
S.M.A.R.T-Self-Monitoring, Analysis and Reporting Technology system counts the total number of errors in the
entire HDD fixed by ECC and the total number of remappings, as the occurrence of many such errors may predict
HDD failure.
Future development
HDD areal densities have shown a long term compound annual growth rate not substantively different from Moore's
Law, most recently in the range of 20-25% annually, with desktop 3.5" drives estimated to hit 12 TB around
2016.[27] New magnetic storage technologies are being developed to support higher areal density growth and
maintain the competitiveness of HDDs with potentially competitive products such as flash memory-based solid-state
drives (SSDs). These new HDD technologies include:
Heat-assisted magnetic recording (HAMR)[28][29]
Bit-patterned recording (BPR)[30]
Current Perpendicular to Plane giant magnetoresistance (CPP/GMR) heads[27]
Shingled Write[27]
With these new technologies the relative position of HDDs and SSDs with regard to their cost and performance is not
projected to change through 2016.[27]
Capacity
The capacity of an HDD reported to an end user by the operating system is less than the amount stated by a drive or
system manufacturer due to amongst other things, different units of measuring capacity, capacity consumed by the file
system and/or redundancy.
Calculation

Because modern disk drives appear to their interface as a contiguous set of logical blocks their gross capacity can be
calculated by multiplying the number of blocks by the size of the block. This information is available from the
manufacturer's specification and from the drive itself through use of special utilities invoking low level
commands.[31][32]
The gross capacity of older HDDs can be calculated by multiplying for each zone of the drive the number of cylinders
by the number of heads by the number of sectors/zone by the number of bytes/sector (most commonly 512) and then
summing the totals for all zones. Some modern SATA drives will also report cylinder-head-sector (C/H/S) values to
the CPU but they are no longer actual physical parameters since the reported numbers are constrained by historic
operating-system interfaces.
The old C/H/S scheme has been replaced by logical block addressing. In some cases, to try to "force-fit" the C/H/S
scheme to large-capacity drives, the number of heads was given as 64, although no modern drive has anywhere near
32 platters.
Redundancy
In modern HDDs, spare capacity for defect management is not included in the published capacity; however in many
early HDDs a certain number of sectors were reserved for spares, thereby reducing capacity available to end users.
In some systems, there may be hidden partitions used for system recovery that reduce the capacity available to the
end user.
For RAID subsystems, data integrity and fault-tolerance requirements also reduce the realized capacity. For example,
a RAID1 subsystem will be about half the total capacity as a result of data mirroring. RAID5 subsystems with x
drives, would lose 1/x of capacity to parity. RAID subsystems are multiple drives that appear to be one drive or more
drives to the user, but provides a great deal of fault-tolerance. Most RAID vendors use some form of checksums to
improve data integrity at the block level. For many vendors, this involves using HDDs with sectors of 520 bytes per
sector to contain 512 bytes of user data and eight checksum bytes or using separate 512-byte sectors for the
checksum data.[33]
File system use
Main article: Disk formatting
The presentation of an HDD to its host is determined by its controller. This may differ substantially from the drive's
native interface particularly in mainframes or servers.
Modern HDDs, such as SAS[31] and SATA[32] drives, appear at their interfaces as a contiguous set of logical blocks;
typically 512 bytes long but the industry is in the process of changing to 4,096-byte logical blocks; see Advanced
Format.[34]
The process of initializing these logical blocks on the physical disk platters is called low level formatting which is
usually performed at the factory and is not normally changed in the field.[note 5]
High level formatting then writes the file system structures into selected logical blocks to make the remaining logical
blocks available to the host OS and its applications.[35] The operating system file system uses some of the disk space
to organize files on the disk, recording their file names and the sequence of disk areas that represent the file. Examples

Unit prefixes[36][37]
Advertised capacity by
manufacturer (using
decimal multiples)
Expected capacity by
consumers in class
action (using
binary multiples)
Reported capacity
Windows
(using
binary
multiples)
Mac OS X
10.6+ (using
decimal
multiples)With
prefix
Bytes Bytes Diff.
100 MB 100,000,000 104,857,600 4.86% 95.4 MB 100 MB
100 GB 100,000,000,000 107,374,182,400 7.37% 93.1 GB,
95,367 MB
100 GB
1 TB 1,000,000,000,000 1,099,511,627,776 9.95% 931 GB,
953,674 MB
1,000 GB,
1,000,000 MB
of data structures stored on disk to retrieve files include the file allocation table (FAT) in the MS-DOS file system and
inodes in many UNIX file systems, as well as other operating system data structures. As a consequence not all the
space on an HDD is available for user files. This file system overhead is usually less than 1% on drives larger than 100
MB.
Units
See also: Binary prefix
The total capacity of HDDs is
given by manufacturers in
megabytes (1 MB = 1,000,000
bytes), gigabytes
(1 GB = 1,000,000,000 bytes)
or terabytes
(1 TB = 1,000,000,000,000
bytes).[36][38][39][40][41][42] This
numbering convention, where
prefixes like mega- and giga-
denote powers of 1,000, is also
used for data transmission rates
and DVD capacities. However,
the convention is different from
that used by manufacturers of
memory (RAM, ROM) and CDs, where prefixes like kilo- and mega- mean powers of 1,024.
The practice of using prefixes assigned to powers of 1,000 within the HDD and computer industries dates back to the
early days of computing.[43] By the 1970s million, mega and M were consistently being used in the powers of 1,000
sense to describe HDD capacity.[44][45][46]
Computers do not internally represent HDD or memory capacity in powers of 1,024; reporting it in this manner is just
a convention.[47] Microsoft Windows uses the powers of 1,024 convention when reporting HDD capacity, thus an
HDD offered by its manufacturer as a 1 TB drive is reported by these OSes as a 931 GB HDD. Mac OS X 10.6
("Snow Leopard"), uses powers of 1,000 when reporting HDD capacity.
In the case of "mega-", there is a nearly 5% difference between the powers of 1,000 definition and the powers of
1,024 definition. Furthermore, the difference is compounded by 2.4% with each incrementally larger prefix (gigabyte,
terabyte, etc.). The discrepancy between the two conventions for measuring capacity was the subject of several class
action suits against HDD manufacturers. The plaintiffs argued that the use of decimal measurements effectively misled
consumers[48][49] while the defendants denied any wrongdoing or liability, asserting that their marketing and
advertising complied in all respects with the law and that no class member sustained any damages or injuries.[50]
In December 1998, standards organizations addressed these dual definitions of the conventional prefixes by
standardizing on unique binary prefixes and prefix symbols to denote multiples of 1,024, such as "mebibyte (MiB)",
which exclusively denotes 220 or 1,048,576 bytes.[51] This standard has seen little adoption by the computer industry,
and the conventionally prefixed forms of "byte" continue to denote slightly different values depending on
context.[52][53]

Past and present HDD form factors
Form factor Status
Length
[mm]
Width
[mm]
Height [mm]
Largest
capacity
Platters
(max)
Capacity
Per platter
[GB]
3.5" Current 146 101.6 19 or 25.4
4 TB[54][note 6]
(2011)
5 1000
2.5" Current 100 69.85
5,[55] 7, 9.5,[note 7]
12.5, or 15
2 TB[56][note 8]
(2012)
4 694[57]
1.8" Current 71 54 5 or 8
320 GB[58][note 9]
(2009)
2 220 [59]
8" Obsolete 362 241.3 117.5
5.25" FH Obsolete 203 146 82.6 47 GB[60] (1998) 14 3.36
5.25" HH Obsolete 203 146 41.4
19.3 GB[61]
(1998)
4[note 10] 4.83
1.3" Obsolete 43 40 GB[62] (2007) 1 40
1"
(CFII/ZIF/IDE-
Flex)
Obsolete 42 20 GB (2006) 1 20
0.85" Obsolete 32 24 5
8 GB[63][64]
(2004)
1 8
5¼" full height 110 MB HDD; 2½"
(63.5 mm) 6,495 MB HDD
Form factors
Mainframe and minicomputer hard disks were of widely varying
dimensions, typically in free standing cabinets the size of washing machines
or designed to fit a 19" rack. In 1962, IBM introduced its model 1311
disk, which used 14 inch (nominal size) platters. This became a standard
size for mainframe and minicomputer drives for many years.[65] Such large
platters were never used with microprocessor-based systems.
With increasing sales of microcomputers having built in floppy-disk drives
(FDDs), HDDs that would fit to the FDD mountings became desirable.
Thus HDD Form factors, initially followed those of 8-inch, 5.25-inch, and
3.5-inch floppy disk drives. Because there were no smaller floppy disk
drives, smaller HDD form factors developed from product offerings or
industry standards.
8 inch
9.5 in × 4.624 in × 14.25 in (241.3 mm × 117.5 mm × 362 mm). In 1979, Shugart Associates' SA1000 was
the first form factor compatible HDD, having the same dimensions and a compatible interface to the 8" FDD.
5.25 inch

2.5" SATA HDD from a
Sony VAIO laptop
Six HDDs with 8", 5.25", 3.5", 2.5",
1.8", and 1" hard disks with a ruler to
show the length of platters and read-
write heads
5.75 in × 3.25 in × 8 in (146.1 mm × 82.55 mm × 203 mm). This smaller form factor, first used in an HDD by
Seagate in 1980,[66] was the same size as full-height 5 1⁄4-inch-diameter (130 mm) FDD, 3.25-inches high.
This is twice as high as "half height"; i.e., 1.63 in (41.4 mm). Most desktop models of drives for optical
120 mm disks (DVD, CD) use the half height 5¼" dimension, but it fell out of fashion for HDDs. The Quantum
Bigfoot HDD was the last to use it in the late 1990s, with "low-profile"
(≈25 mm) and "ultra-low-profile" (≈20 mm) high versions.
3.5 inch
4 in × 1 in × 5.75 in (101.6 mm × 25.4 mm × 146 mm) = 376.77344 cm³.
This smaller form factor is similar to that used in an HDD by Rodime in
1983,[67] which was the same size as the "half height" 3½" FDD, i.e.,
1.63 inches high. Today, the 1-inch high ("slimline" or "low-profile") version
of this form factor is the most popular form used in most desktops.
2.5 inch
2.75 in × 0.275–0.59 in × 3.945 in (69.85 mm × 7–15 mm × 100 mm) =
48.895–104.775 cm3. This smaller form factor was introduced by PrairieTek
in 1988;[68] there is no corresponding FDD. It came to be widely used for
HDDs in mobile devices (laptops, music players, etc.) and for solid-state
drives (SSDs), by 2008 replacing some 3.5 inch enterprise-class drives.[69]
It is also used in the PlayStation 3[70] and Xbox 360[citation needed] video
game consoles. Drives 9.5 mm high became an unofficial standard for all
except the largest-capacity laptop drives (usually having two platters
inside); 12.5 mm-high drives, typically with three platters, are used
for maximum capacity, but will not fit most laptop computers.
Enterprise-class drives can have a height up to 15 mm.[71] Seagate
released a 7mm drive aimed at entry level laptops and high end
netbooks in December 2009.[72] Western Digital released on April
23, 2013 a hard drive 5 mm in height specifically aimed at
UltraBooks.[73]
1.8 inch
54 mm × 8 mm × 71 mm = 30.672 cm³. This form factor, originally
introduced by Integral Peripherals in 1993, evolved into the ATA-7
LIF with dimensions as stated. For a time it was increasingly used in
digital audio players and subnotebooks, but its popularity decreased
to the point where this form factor is increasingly rare and only a
small percentage of the overall market.[74]
1 inch
42.8 mm × 5 mm × 36.4 mm. This form factor was introduced in 1999 as IBM's Microdrive to fit inside a CF
Type II slot. Samsung calls the same form factor "1.3 inch" drive in its product literature.[75]
0.85 inch
24 mm × 5 mm × 32 mm. Toshiba announced this form factor in January 2004[76] for use in mobile phones
and similar applications, including SD/MMC slot compatible HDDs optimized for video storage on 4G
handsets. Toshiba manufactured a 4 GB (MK4001MTD) and an 8 GB (MK8003MTD) version[77] and holds
the Guinness World Record for the smallest HDD.[78]

As of 2012, 2.5-inch and 3.5-inch hard disks were the most popular sizes.
By 2009 all manufacturers had discontinued the development of new products for the 1.3-inch, 1-inch and 0.85-inch
form factors due to falling prices of flash memory,[79][80] which has no moving parts.
While these sizes are customarily described by an approximately correct figure in inches, actual sizes have long been
specified in millimeters.
Performance characteristics
Main article: Hard disk drive performance characteristics
Time to access data
The factors that limit the time to access the data on an HDD are mostly related to the mechanical nature of the rotating
disks and moving heads. Seek time is a measure of how long it takes the head assembly to travel to the track of the
disk that contains data. Rotational latency is incurred because the desired disk sector may not be directly under the
head when data transfer is requested. These two delays are on the order of milliseconds each. The bit rate or data
transfer rate (once the head is in the right position) creates delay which is a function of the number of blocks
transferred; typically relatively small, but can be quite long with the transfer of large contiguous files. Delay may also
occur if the drive disks are stopped to save energy.
An HDD's Average Access Time is its average Seek time which technically is the time to do all possible seeks
divided by the number of all possible seeks, but in practice is determined by statistical methods or simply
approximated as the time of a seek over one-third of the number of tracks.[81]
Defragmentation is a procedure used to minimize delay in retrieving data by moving related items to physically
proximate areas on the disk.[82] Some computer operating systems perform defragmentation automatically. Although
automatic defragmentation is intended to reduce access delays, performance will be temporarily reduced while the
procedure is in progress.[83]
Time to access data can be improved by increasing rotational speed (thus reducing latency) and/or by reducing the
time spent seeking. Increasing areal density increases throughput by increasing data rate and by increasing the amount
of data under a set of heads, thereby potentially reducing seek activity for a given amount of data. Based on historic
trends, analysts predict a future growth in HDD areal density (and therefore capacity) of about 40% per year.[84] The
time to access data has not kept up with throughput increases, which themselves have not kept up with growth in
storage capacity.
Seek time
Average seek time ranges from 3 ms[85] for high-end server drives, to 15 ms for mobile drives, with the most
common mobile drives at about 12 ms[86] and the most common desktop type typically being around 9 ms. The first
HDD had an average seek time of about 600 ms;[2] by the middle 1970s HDDs were available with seek times of
about 25 ms.[87] Some early PC drives used a stepper motor to move the heads, and as a result had seek times as
slow as 80–120 ms, but this was quickly improved by voice coil type actuation in the 1980s, reducing seek times to
around 20 ms. Seek time has continued to improve slowly over time.

Some desktop and laptop computer systems allow the user to make a tradeoff between seek performance and drive
noise. Faster seek rates typically require more energy usage to quickly move the heads across the platter, causing
louder noises from the pivot bearing and greater device vibrations as the heads are rapidly accelerated during the start
of the seek motion and decelerated at the end of the seek motion. Quiet operation reduces movement speed and
acceleration rates, but at a cost of reduced seek performance.
Latency
Latency is the delay for the rotation of the disk to bring the required disk sector under the read-write mechanism. It
depends on rotational speed of a disk, measured in revolutions per minute (rpm). Average rotational latency is shown
in the table below, based on the statistical relation that the average latency in milliseconds for such a drive is one-half
the rotational period.
Rotational speed
[rpm]
Average latency
[ms]
15,000 2
10,000 3
7,200 4.16
5,400 5.55
4,800 6.25
Data transfer rate
As of 2010, a typical 7,200-rpm desktop HDD has a sustained "disk-to-buffer" data transfer rate up to 1,030
Mbits/sec.[88] This rate depends on the track location; the rate is higher for data on the outer tracks (where there are
more data sectors per rotation) and lower toward the inner tracks (where there are fewer data sectors per rotation);
and is generally somewhat higher for 10,000-rpm drives. A current widely used standard for the "buffer-to-computer"
interface is 3.0 Gbit/s SATA, which can send about 300 megabyte/s (10-bit encoding) from the buffer to the
computer, and thus is still comfortably ahead of today's disk-to-buffer transfer rates. Data transfer rate (read/write)
can be measured by writing a large file to disk using special file generator tools, then reading back the file. Transfer
rate can be influenced by file system fragmentation and the layout of the files.[82]
HDD data transfer rate depends upon the rotational speed of the platters and the data recording density. Because
heat and vibration limit rotational speed, advancing density becomes the main method to improve sequential transfer
rates. Higher speeds require more power absorbed by the electric engine, which hence warms up more. While areal
density advances by increasing both the number of tracks across the disk and the number of sectors per track, only
the latter increases the data transfer rate for a given rpm. Since data transfer rate performance only tracks one of the
two components of areal density, its performance improves at a lower rate.[citation needed]
Other considerations
Other performance considerations include power consumption, audible noise, and shock resistance.
Access and interfaces

Inner view of a 1998 Seagate HDD
which used Parallel ATA interface
Main article: Hard disk drive interface
HDDs are accessed over one of a number of bus types, including as of
2011 parallel ATA (PATA, also called IDE or EIDE; described before the
introduction of SATA as ATA), Serial ATA (SATA), SCSI, Serial
Attached SCSI (SAS), and Fibre Channel. Bridge circuitry is sometimes
used to connect HDDs to buses with which they cannot communicate
natively, such as IEEE 1394, USB and SCSI.
Modern HDDs present a consistent interface to the rest of the computer,
no matter what data encoding scheme is used internally. Typically a DSP in
the electronics inside the HDD takes the raw analog voltages from the read head and uses PRML and Reed–
Solomon error correction[89] to decode the sector boundaries and sector data, then sends that data out the standard
interface. That DSP also watches the error rate detected by error detection and correction, and performs bad sector
remapping, data collection for Self-Monitoring, Analysis, and Reporting Technology, and other internal tasks.
Modern interfaces connect an HDD to a host bus interface adapter (today typically integrated into the "south bridge")
with one data/control cable. Each drive also has an additional power cable, usually direct to the power supply unit.
Small Computer System Interface (SCSI), originally named SASI for Shugart Associates System Interface,
was standard on servers, workstations, Commodore Amiga, Atari ST and Apple Macintosh computers
through the mid-1990s, by which time most models had been transitioned to IDE (and later, SATA) family
disks. The range limitations of the data cable allows for external SCSI devices.
Integrated Drive Electronics (IDE), later standardized under the name AT Attachment (ATA, with the alias P-
ATA or PATA (Parallel ATA) retroactively added upon introduction of SATA) moved the HDD controller
from the interface card to the disk drive. This helped to standardize the host/contoller interface, reduce the
programming complexity in the host device driver, and reduced system cost and complexity. The 40-pin
IDE/ATA connection transfers 16 bits of data at a time on the data cable. The data cable was originally 40-
conductor, but later higher speed requirements for data transfer to and from the HDD led to an "ultra DMA"
mode, known as UDMA. Progressively swifter versions of this standard ultimately added the requirement for
an 80-conductor variant of the same cable, where half of the conductors provides grounding necessary for
enhanced high-speed signal quality by reducing cross talk.
EIDE was an unofficial update (by Western Digital) to the original IDE standard, with the key improvement
being the use of direct memory access (DMA) to transfer data between the disk and the computer without the
involvement of the CPU, an improvement later adopted by the official ATA standards. By directly transferring
data between memory and disk, DMA eliminates the need for the CPU to copy byte per byte, therefore
allowing it to process other tasks while the data transfer occurs.
Fibre Channel (FC) is a successor to parallel SCSI interface on enterprise market. It is a serial protocol. In
disk drives usually the Fibre Channel Arbitrated Loop (FC-AL) connection topology is used. FC has much
broader usage than mere disk interfaces, and it is the cornerstone of storage area networks (SANs). Recently
other protocols for this field, like iSCSI and ATA over Ethernet have been developed as well. Confusingly,
drives usually use copper twisted-pair cables for Fibre Channel, not fibre optics. The latter are traditionally
reserved for larger devices, such as servers or disk array controllers.
Serial Attached SCSI (SAS). The SAS is a new generation serial communication protocol for devices
designed to allow for much higher speed data transfers and is compatible with SATA. SAS uses a mechanically
identical data and power connector to standard 3.5-inch SATA1/SATA2 HDDs, and many server-oriented
SAS RAID controllers are also capable of addressing SATA HDDs. SAS uses serial communication instead

Close-up HDD head resting on disk
platter; its mirror reflection is visible
on the platter surface
of the parallel method found in traditional SCSI devices but still uses SCSI commands.
Serial ATA (SATA). The SATA data cable has one data pair for differential transmission of data to the device,
and one pair for differential receiving from the device, just like EIA-422. That requires that data be transmitted
serially. A similar differential signaling system is used in RS485, LocalTalk, USB, FireWire, and differential
SCSI.
Integrity and failure
Main articles: Hard disk drive failure and Data recovery
Due to the extremely close spacing between the heads and the disk surface,
HDDs are vulnerable to being damaged by a head crash—a failure of the
disk in which the head scrapes across the platter surface, often grinding
away the thin magnetic film and causing data loss. Head crashes can be
caused by electronic failure, a sudden power failure, physical shock,
contamination of the drive's internal enclosure, wear and tear, corrosion, or
poorly manufactured platters and heads.
The HDD's spindle system relies on air pressure inside the disk enclosure to
support the heads at their proper flying height while the disk rotates.
HDDs require a certain range of air pressures in order to operate properly.
The connection to the external environment and pressure occurs through a
small hole in the enclosure (about 0.5 mm in breadth), usually with a filter
on the inside (the breather filter).[90] If the air pressure is too low, then
there is not enough lift for the flying head, so the head gets too close to the disk, and there is a risk of head crashes
and data loss. Specially manufactured sealed and pressurized disks are needed for reliable high-altitude operation,
above about 3,000 m (9,800 ft).[91] Modern disks include temperature sensors and adjust their operation to the
operating environment. Breather holes can be seen on all disk drives—they usually have a sticker next to them,
warning the user not to cover the holes. The air inside the operating drive is constantly moving too, being swept in
motion by friction with the spinning platters. This air passes through an internal recirculation (or "recirc") filter to
remove any leftover contaminants from manufacture, any particles or chemicals that may have somehow entered the
enclosure, and any particles or outgassing generated internally in normal operation. Very high humidity for extended
periods can corrode the heads and platters.
For giant magnetoresistive (GMR) heads in particular, a minor head crash from contamination (that does not remove
the magnetic surface of the disk) still results in the head temporarily overheating, due to friction with the disk surface,
and can render the data unreadable for a short period until the head temperature stabilizes (so called "thermal
asperity", a problem which can partially be dealt with by proper electronic filtering of the read signal).
When a mechanical hard disk requires repairs, the easiest method is to replace the circuit board using an identical
hard disk, provided it is the circuit board that has malfunctioned. In the case of read-write head faults, they can be
replaced using specialized tools in a dust-free environment. If the disk platters are undamaged, they can be transferred
into an identical enclosure and the data can be copied or cloned onto a new drive. In the event of disk-platter failures,
disassembly and imaging of the disk platters may be required.[92] For logical damage to file systems, a variety of tools,
including fsck on UNIX-like systems and CHKDSK on Windows, can be used for data recovery. Recovery from
logical damage can require file carving.

Toshiba 1 TB 2.5" external USB 2.0 HDD
3.0 TB 3.5" Seagate FreeAgent GoFlex
plug and play external USB 3.0-compatible
drive (left), 750 GB 3.5" Seagate
Technology push-button external USB 2.0
drive (right), and a 500 GB 2.5" generic
brand plug and play external USB 2.0 drive
(front).
A 2011 summary of research into SSD and magnetic disk failure patterns by Tom's Hardware summarized research
findings as follows:[93]
1. MTBF does not indicate reliability; the annualized failure rate is higher and usually more relevant.
2. Magnetic disks do not have a specific tendency to fail during early use, and temperature only has a minor
effect; instead, failure rates steadily increase with age.
3. SMART warns of mechanical issues but not other issues affecting reliability, and is therefore not a reliable
indicator of condition.
4. Failure rates of drives sold as "enterprise" and "consumer" are "very much similar", although customized for
their different environments.
5. In drive arrays, one drive's failure significantly increases the short-term chance of a second drive failing.
External removable drives
External removable HDDs[note 11] typically connect via USB. Plug
and play drive functionality offers system compatibility and features
large storage options and portable design. External HDDs are
available in 2.5" and 3.5" sizes, and as of March 2012 their capacities
generally range from 160GB to 2TB. Common sizes are 160GB,
250GB, 320GB, 500GB, 640GB, 750GB, 1TB, and 2TB.[94][95]
External HDDs are available as preassembled integrated products or
may be assembled by combining an external enclosure (with USB or
other interface) with a separately purchased drive.
Features such as biometric security or multiple interfaces are available
at a higher cost.[96]
External hard drives generally have a slower transfer rate than that of
an internally mounted hard drive connecting through SATA.
Market segments
Desktop HDDs typically store between 60 GB and 4 TB and
rotate at 5,400 to 10,000 rpm, and have a media transfer rate
of 0.5 Gbit/s or higher (1 GB = 109 bytes; 1 Gbit/s = 109
bit/s). As of September 2011, the highest capacity consumer
HDDs store 4 TB.[54]
Mobile HDDs or laptop HDDs, smaller than their desktop and
enterprise counterparts, tend to be slower and have lower
capacity. Mobile HDDs spin at 4,200 rpm, 5,200 rpm, 5,400
rpm, or 7,200 rpm, with 5,400 rpm being typical. 7,200 rpm
drives tend to be more expensive and have smaller capacities,
while 4,200 rpm models usually have very high storage capacities. Because of smaller platter(s), mobile HDDs
generally have lower capacity than their greater desktop counterparts.
Enterprise HDDs are typically used with multiple-user computers running enterprise software. Examples are:

Diagram of HDD manufacturer
consolidation
transaction processing databases, internet infrastructure (email, webserver, e-commerce), scientific computing
software, and nearline storage management software. Enterprise drives commonly operate continuously
("24/7") in demanding environments while delivering the highest possible performance without sacrificing
reliability. Maximum capacity is not the primary goal, and as a result the drives are often offered in capacities
that are relatively low in relation to their cost.[97] The fastest enterprise HDDs spin at 10,000 or 15,000 rpm,
and can achieve sequential media transfer speeds above 1.6 Gbit/s[98] and a sustained transfer rate up to
1 Gbit/s.[98] Drives running at 10,000 or 15,000 rpm use smaller platters to mitigate increased power
requirements (as they have less air drag) and therefore generally have lower capacity than the highest capacity
desktop drives. Enterprise HDDs are commonly connected through Serial Attached SCSI (SAS) or Fibre
Channel (FC). Some support multiple ports, so they can be connected to a redundant host bus adapter. They
can be reformatted with sector sizes larger than 512 bytes (often 520, 524, 528 or 536 bytes). The additional
storage can be used by hardware RAID cards or to store a Data Integrity Field.
Consumer electronics HDDs include drives embedded into digital video recorders and automotive vehicles.
The former are configured to provide a guaranteed streaming capacity, even in the face of read and write
errors, while the latter are built to resist larger amounts of shock.
The exponential increases in disk space and data access speeds of HDDs have enabled consumer products that
require large storage capacities, such as digital video recorders and digital audio players.[99] In addition, the
availability of vast amounts of cheap storage has made viable a variety of web-based services with extraordinary
capacity requirements, such as free-of-charge web search, web archiving, and video sharing (Google, Internet
Archive, YouTube, etc.).
Manufacturers and sales
See also: History of hard disk drives and List of defunct hard disk
manufacturers
More than 200 companies have manufactured HDDs over time. But
consolidations have concentrated production into just three manufacturers
today: Western Digital, Seagate, and Toshiba.
Worldwide revenues for HDDs shipments are expected to reach $33 billion
in 2013, down about 12% from $37.8 billion in 2012. This corresponds to
a 2013 unit shipment forecast of 552 million compared to 577 million units
in 2012 and 624 million units in 2011. The estimated 2013 market shares
are about 40-45% each for Seagate and Western Digital and 13-16% for
Toshiba[100]
Icons
HDDs are traditionally symbolized as a stylized stack of platters or as a
cylinder and are found in diagrams, or on lights to indicate HDD access. In
most modern operating systems, HDDs are represented by an illustration or photograph of the drive enclosure.

HDDs are commonly symbolized with a drive
icon
RAID diagram icon symbolizing the array of
disks
See also
Automatic acoustic management
Cleanroom
Click of death
Data erasure
Drive mapping
Hybrid drive
S.M.A.R.T.
Solid-state drive
Write precompensation
Notes
1. ^ This is the original filing date of the application which led to US Patent 3,503,060, generally accepted as the
definitive disk drive patent.[1]
2. ^ Further terms used to describe hard disk drives include hard drive, hard disk, disk drive, disk file, direct access
storage device (DASD), fixed disk, CKD disk, and Winchester disk drive (after the IBM 3340). The term DASD
includes other devices besides disks.
3. ^ Initially gamma iron oxide particles in an epoxy binder, the recording layer in a modern HDD typically is domains of
a granular Cobalt-Chrome-Platinum based alloy physically isolated by an oxide to enable perpendicular recording.[12]
4. ^ Historically a variety of run-length limited codes have been used in magnetic recording including for example, codes
named FM, MFM and GCR which are no longer used in modern HDDs.
5. ^ See: Low-Level Formatting (http://www.pcguide.com/ref/hdd/geom/formatLow-c.html). However, some
enterprise SAS drives have other block sizes such as 520, 524 and 528 bytes which can be changed in the field.
6. ^ 750 GB for IDE-based barebone.
7. ^ Most common.
10. ^ The Quantum Bigfoot TS used a maximum of 3 platters, other earlier and lower capacity product used up to 4
platters in a 5.25" HH form factor, e.g., Microscience HH1090 circa 1989.
11. ^ These differ from removable disk media, e.g., disk packs or data modules, in that they include, for example,
actuators, drive elctronics, motors.

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