In secondary memory we stores data permanently for future purpose. It has much large capacity than the main memory. Once data or information stores in this memory do not erase of loss until the user delete it. It stores the system software, large data files, useful packages etc. computer's secondary memory is unlimited. It is also non-volatile in nature. The secondary storage capacity can be increased by adding Hard Disks as per the requirement.
MAGNETIC TAPE :
This is an external memory. Magnetic tape looks like the audio cassette tape, the data stores in that tape and the tape is coated with magnetizable recording material. The tape used in a computer can be 12.5 mm to 25 mm wide plastic film type and 500 meters to 1200 meters like audio recording tapes, computer tape can be erased and reused indefinitely.
FLOPPY DISK :
Floppy disk is a rounded Mylar or Vinyl plastic materials with magnetic coating on one or both sides and sealed permanently in a plastic jacket to protect them from dust and scratches. There are two standard size for these disks. These are 8 inches, 5.25 inches and 3.5 inches floppies come in two density, double density and high density. The 8" diskette is obsolete now.
Depending on the density and the number of sides used, we have the following types of floppies.
DSSD - Double Sided Single Density
DSDD - Double Sided Double Density
DSHD - Double Sided High Density
The capacity of floppy disk is 1.44 MB.
HARD DISK :
Hard disk is the secondary memory which stores huge amount of data. It is more stable and protected. The Hard Disk is made up of collection of diskettes known as platters. It is sealed in a vacuum container, and located inside the system unit. When the tracks having the same track number, on the different platters of a hard disk are joined together, they form a cylinder. Now hard disks are coming with a capacity of GB (Gigabyte) and TB (Terabyte).
CD (Compact Disk) :
This memory device is use to store data for a long time and it easily move from one point to another. CD-ROM is just one form of optical storage. In addition to this prerecorded read only technology, there are laser based writable optical storage devices also appearing in the market with Write Once Read Many or WORM system, users record their data on blank optical disks, Now a days Re-Writable CD (RW-CD) also available, in which you can Write and Erase the data if you want.
DVD (Digital Video Disk) : Digital Video Disk (DVD) is structurally same as CD, but it has the Higher capacity than CD, to write or burn DVD you need DVD Drive to be fixed in CPU. DVD also available with RW (Re-Writable) facility.