1. EXTERNAL STORAGE DEVICES

2. Magnetic Tapes
Magnetic tapes are sequential access secondary storage media.
Types of magnetic tape include:
Reel-to-reel tapes
Cassette tapes
Cartridge tapes

3. Reel-to-reel tapes
Large computers use tape drives to store and read data on reels of tape.
A reel of magnetic tape is half inch in width and can store 1,600 to 6,000 characters per inch along its 2,400 feet length.

4. Cassette and cartridge tapes
Tape drives attached to early personal computers used cassette tapes similar to the familiar audio cassettes.
Cassettes and reels are now being replaced by cartridge tapes.
These cartridges are self-contained in hard plastic cells that are easy to mount in a tape drive and store when not in use.
They can hold between 40 MB and 1 0 GB of data.

5. Magnetic Disks
Magnetic disks are the most commonly used storage media because they support direct access to data.
Magnetic disk products include:
Floppy disks
Winchester disks
Removable disk packs
Disk cartridges

6. Floppy disk
The most common storage medium used on small computers today is a floppy disk.
It is a flexible plastic disk coated with magnetic material and looks like a phonograph record.
Information can be recorded or read by inserting it into a disk drive connected to the computer.

7. Floppy disk
The disks are permanently encased in stiff paper jackets for protection and easy handling.
An opening is provided in the jacket to facilitate reading and writing of information.
One is 5.25 inch and the other is 3.5 inch floppy disk.

8. DOs and DONTs for Floppies
Store them in a proper storage container
Carefully insert the disk into the drive, gently and firmly in its place
Close the drive doors to avoid the entry of dust

9. DOs 4. Turn the power on before inserting a disk
5. Store them upright when not in use
6. Write identification label separately and then place them on the disk

10. DONTs Do not touch the surface Do not fold or bend them
Do not write on the surface
Do not keep disks near a magnetic field or heat surface

11. DONTs 5. Never use paper clips or rubber bands on disks
6. Never turn the power off before removing a disk from the drive
7. Do not use any solutions to clean disks

12. Winchester disk
magnetic media suitable for storing large volumes of information is the hard disk, popularly known as the Winchester disk.
They can hold much larger volume of information than floppies.
They are very fast in reading and writing

13. Removable disk pack
These are similar to Winchester disks but are removable
Because disk packs can be inserted and removed, we can use any number of disk packs with a drive.
They are portable and interchangeable.

14. Disk cartridge
Disk cartridges are small hard disks that can be mounted and removed from disk drives.
They are compact, measuring only 3.5 inches in diameter.
store more than 20 MB of data.
They are portable and interchangeable.

15. Optical Disks
Optical disks differ from magnetic disks by the way data is stored and read.
In optical disks, a laser beam is used to write and read data on this reflective disks.
Optical disks support direct access to data and have more storage capacity than magnetic disks.

16. Optical disks
Optical disks are popularly known as Compact Disks (CDs).
Optical disks are available in
CD-ROM (Compact disk-read-only memory)
CD-R (Compact disk-recordable)
Magneto-optical (MO) disks
Write once, read many (WORM) disks

17. CD-ROM disks These are used to distribute large volume of data.
Online encyclopaedia or multimedia presentations.
Computer programs and user manuals are often distributed on CD-ROMS.

18. CD-R disks These are recordable compact disks.
They are less expensive and are used to record the contents of a CD-ROM to create master copies.
They are also used by business houses to store certain permanent records.

19. Magneto-optical (MO) disks
In these disks, bit patterns are aligned magnetically an the disk and therefore they are known as magneto-optical disks.
They are erasable disks.
We can overwrite data on these disks. They are the most flexible type of optical disks.

20. Write once, read many (WORM) disks
Writable optical technology
As with the CD, once data have been etched into the surface of the WORM disk, they cannot be changed.
WORM disks are used for storing permanent records.