Secondary storage devices such as hard disks, CDs, USB drives, and Blu-ray discs are used for bulk storage of data with large capacities and transfer data to main memory for processing. Hard disks provide fast storage and high capacities, storing data on magnetic disks accessed by read/write heads. CDs and Blu-ray discs use optical technology to store digital data in pits encoded in spiral tracks, with Blu-ray using a blue laser to achieve higher densities. USB drives are portable flash memory devices that can easily transfer data between computers. Secondary storage is cheaper than main memory and has slower access times.

1. SECONDARY STORAGE DEVICES
Secondary storage devices are also known as Auxiliary Storage Devices. The auxiliary storage devices
store system programs, large data files, assemblers, compilers and other programs. In other words
auxiliary storage devices are used for bulk storage of data. The storage capacity of these devices is
unlimited as an empty device can replace
them once the existing device is completely
filled.
The secondary storage is permanent
innature i.e. the information stored in these
devices does not lost unless it is specifically
deleted. Secondary storage devices being
permanent in nature can be used for the
transportationof data from one computer to
another.
Secondary storage devices are
cheaper as compared to the main memory.
The informationstoredinsecondarymemory
are firsttransferredtothe main memory and
then processed by the CPU. The final result
may then be placed in the secondary
memory. It is because of this that the access
time of the secondary memory is
comparatively high.
There are four types of secondary
storage devices which are discussed below.
These are;
 Hard Disk.
 Compact Disc.
 USB (Universal Significant Bit).
 Blu-Ray Disc.
HARD DISK DRIVE
The hard disk drive was primarily developed with the reason to provide a secondary storage device
whichisfast andhave highercapacityand compactnessaswell asthat is muchreliable as a data storage
system.Asitis knownthathard diskdrive is basically an internal. In some cases it can also be external.
The advantage regarding the external hard disk drive is that it is easy to move the data and user
informationfromone place toanother.The otherforms of data transfercan be made on network itself.
But the external harddiskdrive isusedtotransferthe data that isquietlarge enough;and which cannot
be moved over the network.

2. The internal architecture of hard disk consists of four components;
 Chassis.
 Disk platter.
 Head arm.
 Head Actuator.
The chassis is the part of the hard disk which acts
as a base over which other parts of the hard disk are
placed. The disk platter is usually a magnetic material
where all the data or information is stored over. The disk
platterwhichhas the data over it can be accessed with the
helpof a headarm thatmovesaround the platter to access
the data or the information. Head arm is controlled by
anothermajorcomponentof the hard disk that is the Head
Actuator.
The Head Actuator is an electronic device that is
controlled by a rotating motor. It helps the Head arm in
movingall over the hard disk platter. Usually in the earlier
hard diskdrivesthe Head Actuatorthat ispresentinside the harddiskdrive was controlled by a stepper
motor.Presentlythe newerhard disk drives have the commonly used motor as the servo motor that is
used by the Head Actuator.
Hard disk drives have a large capacity to store large amount of data. Today highest capacity
consumer HDDs are 3TB. HDDs have very high speed. The fastest enterprise HDDs spin at 10,000 or
15,000 rpm, andcan achieve sequential mediatransferspeedsabove 1.6Gbit/sanda sustainedtransfer
rate up to 1 Gbit/s. Drives running at 10,000 or 15,000 rpm use smaller platters to migrate increased
power requirements and therefore generally have lower capacity than the highest capacity desktop
drives.
CD-ROM
A Compact Disc (also known as a CD) is an optical disc used to store digital data. It was originally
developedtostore soundrecordingsexclusively,butlateritalsoallowedthe preservationof othertypes
of data. Standard CDs have a diameter of 120 mm and can hold up to 80 minutes of uncompressed
audio(700 MB of data). The Mini CD has various diameters ranging from 60 to 80 mm; they are
sometimes used for CD singles or device drivers, storing up to 24 minutes of audio.
A CD is made from 1.2 mm thick, almost-pure polycarbonate plastic and weighs 15–20 grams.
From the centeroutward,componentsare:the center(spindle) hole,the first-transition area (clamping
ring),the clampingarea(stackingring),the second-transitionarea(mirrorband),the information (data)
area,and the rim. CD data are storedas a seriesof tinyindentationsknownas"pits",encodedina spiral
track moldedintothe topof the polycarbonate layer.The areasbetweenpitsare knownas"lands".Each
pit is approximately 100 nm deep by 500 nm wide, and varies from 850 nm to 3.5 µm in length.
CD-ROM Data Storage
Although the disc media and the drives of the CD and CD-ROM are, in principle, the same, there is a
difference in the way data storage is organized. Two new sectors were defined, Mode 1 for storing
computer data and Mode 2 for compressed audio or video/graphic data.

3. CD-ROM Mode 1
CD-ROMMode 1 is the mode usedforCD-ROMs that carry data and applicationsonly.Inorder to access
the thousandsof data filesthatmaybe presentonthistype of CD, precise addressing is necessary. data
is stored in sectors (the smallest separately addressable block of information), which each hold 2,352
bytes of data, with an additional number of bytes used for error detection and correction, as well as
control structures .There are 75 sectors per second on the disk, which yields a disc capacity of
681,984,000 bytes(650MB) and a single speedtransferrate of 150 Kbps,withhigherratesfor faster CD-
ROM drives.Drive speedisexpressedasmultiplesof the singlespeedtransfer rate, as 2X, 4X, 6X, and so
on.
CD-ROM Mode 2
CD-ROM Mode 2 is used for compressed audio/video information and uses only two layers of error
detectionandcorrection,the same asthe CD-DA.Therefore,all 2,336 bytesof data behind the sync and
header bytes are for user data. Although the sectors of CD-DA, CD-ROM Mode 1 and Mode 2 are the
same size, the amount of data that can be stored varies considerably because of the use of sync and
headerbytes,errorcorrectionanddetection.Itallowsdifferentkindsof data to be mixed together, and
became the basis for CD-ROMXA.
USB DEVICE
In the present time of advanced storage devices USB is another important storage device.
USB stands for ‘UNIVERSAL SERIAL BUS’. A USB drive -- also known as a flash drive or keychain drive --
isa plug-and-playportablestorage device thatusesflashmemoryandislightweightenoughto attach to
a keychain.A USB drive canbe usedinplace of a floppydisk,Zipdrive disk, or CD. When the user plugs
the device into the USB port, the computer's operating system recognizes the device as a removable
drive and assigns it a drive letter.
Unlike mostremovable drives, a USB drive does not require rebooting after it's attached, does
not require batteriesoran external powersupply,.Several manufacturersofferadditional features such
as password protection, and downloadable drivers that allow the device to be compatible with older
systems that do not have USB ports. USB drives are available in capacities ranging up to about 65
gigabytes (GB), depending on manufacturer, in a corresponding range of prices.
Witha USB drive,datacan be retainedforlongperiods when the device is unplugged from the
computer, or when the computer is powered-down with the drive left in. This makes the USB drive
convenient for transferring data between a desktop computer and a notebook computer, or for
personal backup needs.
Blu-ray Disc
Blu-rayDiscis an optical discstorage mediumdesignedtosupersede the standard DVD format. Its main
usesare for storing high-definition video, PlayStation 3 video games, and other data, with up to 25 GB
per single-layered, and 50 GB per dual-layered disc. Although these numbers represent the standard
storage for Blu-rayDiscdrives,the specificationisopen-ended,with the upper theoretical storage limit
left unclear. The discs have the same physical dimensions as standard DVDs and CDs.

4. The name Blu-rayDiscderivesfromthe "blue laser" used to read the disc. While a standard DVD uses a
650 nanometerredlaser,Blu-rayDisc uses a shorter wavelength 405 nm laser, and allows for over five
timesmore data storage onsingle-layerandovertentimesondouble-layerBlu-rayDisc than a standard
DVD. The laser color is called "blue," but is violet to the eye, and is very close to ultraviolet ("black
light").
Blu-Ray Discs can be clustered together in systems such as optical jukeboxes to increase data
storage. This increase of storage can span multiple terabytes and utilize hundreds of Blu-Ray Discs.
These systems are currently the largest storage units using Blu-Ray technology.
Technical specifications:
Type Physical size Single layer capacity Dual layer capacity
Standarddisc size 12 cm 25 GB /23866 MiB /25025314816 B 50 GB / 47732 MiB / 50050629632 B
Mini disc size 8 cm 7.8 GB / 7430 MiB / 7791181824 B 15.6 GB / 14860 MiB / 15582363648 B
Blu-rayDiscusesa "blue" laser,operatingata wavelength of 405 nm,to read andwrite data. The diodes
are GaN (gallium nitride) lasers that produce 405 nm photons directly, that is, without frequency
doublingorothernonlinearoptical mechanisms. Conventional DVDs and CDs use red and near-infrared
lasers, at 650 nm and 780 nm, respectively.
The blue-violet laser's shorter wavelength makes it possible to store more information on a
12 cm CD/DVD-size disc. The minimum "spot size" on which a laser can be focused is limited by
diffraction,anddependsonthe wavelengthof the light and the numerical aperture of the lens used to
focusit.By decreasingthe wavelength,increasingthe numerical aperture from 0.60 to 0.85, and making
the cover layer thinner to avoid unwanted optical effects, the laser beam can be focused to a smaller
spot. This allows more information to be stored in the same area. For Blu-ray Disc, the spot size is
580 nm.In additiontothe optical improvements,Blu-ray Discs feature improvements in data encoding
that further increase the capacity.
Since the Blu-ray Disc data layer is closer to the surface of the disc compared to the DVD
standard, it was at first more vulnerable to scratches. The first discs were housed in cartridges for
protection, resembling Professional Discs introduced by Sony in 2003.
Usinga cartridge wouldincrease the price of an already expensive medium, so hard-coating of
the pickup surface was chosen instead. TDK was the first company to develop a working scratch-
protection coating for Blu-ray Discs. It was named Durabis. In addition, both Sony and Panasonic's
replication methods include proprietary hard-coat technologies. Sony's rewritable media are spin-
coated, using a scratch-resistant and antistatic coating. Verbatim's recordable and rewritable Blu-ray
Discs use their own proprietary hard-coat technology, called Scratch Guard.
Blu-ray Disc specification allows the use of such a layer to meet the required scratch
resistance. DVD media are not required to be scratch-resistant, but since development of the
technology,some companies,suchasVerbatim, implementedhard-coatingfor more expensive lineups
of recordable DVDs.