External memory generally know as secondary storage includes magnetic storage devices such as hard disk, magnetic tape etc. That type of memory is used for store data, information and program for future use. Secondary storage is non-volatile and not direct-ally accessible to the CPU.
Magnetic tape is most popular storage medium for large data storage. They are access and processed sequentially. Magnetic tape medium is a plastic ribbon usually half inch wide and 50 to 2400 feet large. it is coated with a recording material that can be magnetized such as iron oxides.
magnetic disk are most popular on-line secondary storage device. Magnetic disk is a thin circular plate made of metal or plastic and coated on both side with a recording material that can be magnetized such as iron-oxide.
Hard disk is a magnetic disk where we can store computer data. Hard disk hold more data and faster then floppy disk. It can store anywhere from 10 Mb to several Gb. Hard disk consists of several platters. Each platter requires two read/write heads, one for each side there are attached to a single access arm, that can not move independently.It is possible to buy removable hard disk.
A soft magnetic disk is called floppy disk. Floppy disk are slower to access then hard disk but has less storage capacity. They are expensive and portable. ‘The floppy come into two size.
- 5¼ inch. The most common size are 360 k and 1.2 Mb.
- 3½ inch. support disk of 400 k, 800 k and 1.2 Mb.
Optical disk is a medium that consists of a rotating disk coaled with a highly reflective metal or some other material. laser bean technology is used for recording/reading of data on the disk.