What is Secondary Storage (External Storage Device)?

What is Secondary Storage (External Storage Device)?

Floppy diskettes, hard disk, taps and optical disks come under the category of external storage device or ancillary storage device. These devices are very sensitive to environmental conditions (humidity and temperature) as well as to external magnetic fields and need to be stored carefully.

Types of external storage device are as follows:

1.      Floppy disk

2.      Hard disk

3.      Tape

4.      Optical disk

1.      Floppy disk:

It is magnetic storage media, using flat circular platter. Floppy disks are primarily used on PCs . Information on a floppy disk is recorded in the magnetized states of particles of iron oxides evenly placed upon concentric circles known as Tracks. Tracks are further divided into sectors. The difference between the capacities of double density and the quad density diskettes is in the number of tracks per inch (tpi)- 48 tpi and 996 tpi. Respectively, and in the number of sectors per track; usually this is eight or nine for ordinary diskettes and fifteen for high capacity diskettes.

 Physical  size and storage capabilities of disks very.

Type                                                           Size                                       Capacity

Double Density (DD)                             5.25”                                  360 KB

High Density (HD)                                    5.25”                               1.2 MB

Double Density                                      3.5”                                    720 KB

High Density                                          3.5”                                          1.44 MB

Very High Density                                    3.5”                                  2.88 MB

2.      Hard Disk:

It is a non-removable enclosed magnetic disk included in most PCs. It contains a stack of metal platters, each coated with iron oxide, that spin on a spindle and the entire unit is encased in a sealed chamber. A hard disk holds much more information than a floppy disk and is used to store relatively large amounts of data. The physical size and storage capabilities of a hard disk vary. For PCs, the storage capacities vary from 1 GB to about 8-10 GB. On mainframe computers (explained later) the size of hard disk may vary from 8 or 16 GB to several gigabytes (GBs).

Technically microcomputer hard disks are called Winchester disks or fixed disks. These disks spin at an average speed of 3600 rpm (equivalent to 150km/hr) and unlike floppy disks they spin continuously.

Winchester disk is a particular type of hard disk technology that uses a high density of data storage tracks and a very low flying read/write head, requiring a closely controlled operating environment.

3.      Magnetic Tape:

This is plastic tape, usually made of Mylar that is coated with iron oxide, thereby enabling the introduction (writing), retention (memory) and reading of magnetically recorded information. The best use of tape storage is for data that you do not use very often. A tape is a long strip of magnetic material; therefore the tape drive has to write data to it sequentially- one byte after another.

Tapes are sequential access media, which means to get to a particular point on the tape; the tape must go through all the preceding points. In contrast, disks are random access media because a disk drive can access any point at random without passing through intervening points. Therefore, tapes cannot be practically used for data that you need often. They are mainly used for taking backup of hard disk. The tape capacity varies from few hundred kilobytes to several gigabytes.

4.      Optical Disks:

The need for ever greater storage capacities has driven hardware manufacturers to use optical storage technology as the main alternative to magnetic storage. Optical storage techniques make use of the pinpoint precision possible with laser beams.

Types of popular Optical disks are as follows:

a)      CDROM

b)      WORM

c)      CD-R

d)      DVD

a)      CDROM

The most-popular optical storage medium is compact disk, read- only memory. CD-ROM uses the same technology that is used in music CDs. One cannot write data to a CD-ROM but can always read it. The huge amounts of data, such as dictionaries, encyclopedias, medical. legal or other professional reference libraries etc., that normally would not change , is available on CDs nowadays. A single CD can hold up to 650 MB of data. Larger CDs can store up to 20 GB.

b)     WORM                                                                                                                               Due to tremendous capacities of CDs, writable optical devices have been developed. This has resulted in the write once, read many (WORM) DRIVE. AS with CD, once data has been written onto a WROM disk, it cannot be altered. For example, many banks use WORM disks to store a record of each day’s transactions.

c)      Compact Disk-Recordable(CD-R)

It is similar to CD-ROM and can be produced by anyone who has a CD-writer. Data can be recorded only once in a CD-r to produce an audio CD or CD-ROM.

d)      Digital Versatile Disc (DVD)

Digital Versatile Disk or Digital Video Disc, a new type of CD-ROM  that holds a minimum of 4.7 GB enough for a full length movie. The DVD specification supports disk with capacities from 4.7 GB to 17 GB and access rate of 600 Kbps to 1.3 Mbps (Mega Bits per second)