A hard disk drive (HDD; also hard drive, hard disk, or disk drive)
is a device for storing and retrieving digital information, primarily
computer data. It consists of one or more rigid (hence "hard")
rapidly rotating discs (platters) coated with magnetic
material, and with magnetic heads arranged to write data to the
surfaces and read it from them. Hard drives are classified as nonvolatile, random access, digital, magnetic, data storage devices.
Hard disk drives were introduced in 1956 as data storage for an IBM
real-time transaction processing computer and were developed for use
with general purpose mainframe and mini computers. The first IBM
drive, the 350 RAMAC, was approximately the size of two refrigerators
and stored 5 million 6-bit characters on a stack of 50 discs.
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.
As the 1980s began, hard disk drives were a rare and very expensive
additional feature on personal computers (PCs); however by the late
'80s, their cost had been reduced to the point where they were
standard on all but the cheapest PC.
External hard disk drives 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’’.
A hard disk drive records data by magnetizing a thin film of
ferromagnetic material 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 run-length limited encoding,
which determines how the data is represented by the magnetic
The magnetic surface of each platter is conceptually divided into
many small sub-micrometer-sized magnetic regions, referred to as
magnetic domains, (although these are not magnetic domains in a
rigorous physical sense), each of which has a mostly uniform
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.
A typical hard disk drive has two electric motors; a disk motor that
spins the disks and an actuator (motor) that positions the read/write.
The hard drive'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.
Modern disk firmware is capable of scheduling reads and writes
efficiently on the platter surfaces and remapping sectors of the media
which have failed.
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 hard drive, but allow
higher recording densities to be employed without causing
uncorrectable errors, resulting in much larger storage capacity.
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
Due to bit-flipping errors and other issues, perpendicular recording
densities may be supplanted by other magnetic recording
technologies. Toshiba is promoting bit-patterned recording
(BPR), while Xyratex is developing heat-assisted magnetic recording
October 2011: TDK has developed a special laser that heats up a hard
disk's surface with a precision of a few dozen nanometers. TDK also
used the new material in the magnetic head and redesigned its
structure to expand the recording density. This new technology
apparently makes it possible to store one terabyte on one platter and
for the initial hard drive TDK plans to include two platters.
The capacity of an HDD may appear to the end user to be a different
amount than the amount stated by a drive or system manufacturer due
to amongst other things, different units of measuring capacity,
capacity consumed in formatting the drive for use by an operating
system and/or redundancy.
In December 1998, an international standards organization attempted
to address these dual definitions of the conventional prefixes by
proposing 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.
In the over-13 years that have since elapsed, the proposal has seen
little adoption by the computer industry and the conventionally
prefixed forms of “byte” continue to denote slightly different values
depending on context.
Access and interfaces
Hard disk drives 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 hard disk drives to
buses with which they cannot communicate natively, such as
IEEE 1394, USB and SCSI.
Modern hard drives present a consistent interface to the rest of the
computer, no matter what data encoding scheme is used internally.
External removable drives
3.0 TB 3.5" Seagate FreeAgent GoFlex plug and play external USB
3.0-compatible drive (left), 750 GB 3.5" Seagate Technology pushbutton external USB 2.0 drive (right), and a 500 GB 2.5" generic brand
plug and play external USB 2.0 drive (front).
External removable hard disk drives typically connect via USB. Plug
and play drive functionality offers system compatibility, and features
large storage options and portable design.
External hard disk drives 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, 1TB, and
Large storage capacity.
Stores and retrieves data much faster than a floppy disk or CD-ROM.
Data is not lost when you switch off the computer.
Usually fixed inside the computer so cannot get mislaid.
Cheap on a cost per megabyte compared to other storage media.
Hard disks can be replaced and upgraded as necessary.
Can have two hard disks in a machine, one can act as a mirror of the
other and create a back up copy.
Hard disks eventually fail which stops the computer from working.
Regular 'head' crashes can damage the surface of the disk, leading to
loss of data in that sector.
The disk is fixed inside the computer and cannot easily be
transferred to another computer .
In computing and optical disc recording technologies, an optical
disc (OD) is a flat, usually circular disc which encodes binary data
(bits) in the form of pits (binary value of 0 or off, due to lack of
reflection when read) and lands (binary value of 1 or on, due to a
reflection when read) on a special material on one of its flat
Optical discs are usually between 7.6 and 30 cm (3 to 12 in) in
diameter, with 12 cm (4.75 in) being the most common size.
An optical disc is designed to support one of three recording
types: read-only (e.g.: CD and CD-ROM), recordable (writeonce, e.g. CD-R), or re-recordable (rewritable, e.g. CD-RW.
Optical discs are most commonly used for storing music (e.g. for
use in a CD player), video (e.g. for use in a Blu-ray player), or data
and programs for personal computers (PC).
The optical disc was invented in 1958. In 1961 and 1969, David Paul
Gregg registered a patent for the analog optical disc for video
recording. This form of optical disc was a very early form of the DVD
U.S. Patent 3,430,966. It is of special interest that U.S. Patent
4,893,297, filed 1989, issued 1990, generated royalty income for
Pioneer Corporation's DVA until 2007 then encompassing the
CD, DVD, and Blu-ray systems.
Later, in the Netherlands in 1969, Philips Research physicists began
their first optical videodisc experiments at Eindhoven. In 1975, Philips
and MCA began to work together, and in 1978, commercially much too
late, they presented their long-awaited Laserdisc in Atlanta.
In Japan and the U.S., Pioneer succeeded with the videodisc until the
advent of the DVD. In 1979, Philips and Sony, in
consortium, successfully developed the audio compact disc.
In the mid-1990s, a consortium of manufacturers developed the second
generation of the optical disc, the DVD.
Initially, optical discs were used to store music and computer
software. The Laserdisc format stored analog video signals for the
distribution of home video, but commercially lost to the VHS
videocassette format, due mainly to its high cost and non-rerecordability; other first-generation disc formats were designed only to
store digital data and were not initially capable of use as a digital
Compact Disc (CD) and derivatives
Video CD (VCD)
Super Video CD
Double Density Compact Disc (DDCD)
Second-generation optical discs were for storing great amounts of
data, including broadcast-quality digital video. Such discs usually
are read with a visible-light laser (usually red); the shorter
wavelength and greater numerical aperture allow a narrower light
beam, permitting smaller pits and lands in the disc. In the DVD
format, this allows 4.7 GB storage on a standard 12 cm, single-sided,
single-layer disc; alternatively, smaller media, such as the DataPlay
format, can have capacity comparable to that of the larger, standard
compact 12 cm disc.
DVD and derivatives
Digital Video Express (DIVX)
Super Audio CD
Enhanced Versatile Disc
Universal Media Disc
Ultra Density Optical
Third-generation optical discs are in development, meant for
distributing high-definition video and support greater data storage
capacities, accomplished with short-wavelength visible-light lasers
and greater numerical apertures. Blu-ray Disc and HD DVD uses blueviolet lasers and focusing optics of greater aperture, for use with discs
with smaller pits and lands, thereby greater data storage capacity per
layer. In practice, the effective multimedia presentation capacity is
improved with enhanced video data compression codecs such as
H.264/MPEG-4 AVC and VC-1.
Blu-ray Disc (up to 128 GB (quad-layer))
CBHD (a derivative of the discontinued disc format HD DVD)
Forward Versatile Disc
Digital Multilayer Disk or Fluorescent Multilayer Disc
The following formats go beyond the current third-generation
discs and have the potential to hold more than one terabyte (1 TB)
Holographic Versatile Disc
Advantages and Disadvantages of Optical Disk:Advantage:
Easy to scratch.
CD & DVD can hold lots
Easy to break.
Easy to carry
Small in size.
Can get damaged.
Small capacity compared to
some flash/thumb drive.