<Li> Optical disc: A storage medium from which data is read and written by lasers . Optical disks can store much more data--up to 6 gigabytes more than most portable magnetic media, such as floppies . There are three basic types of optical disks: CD / DVD / BD - ROM (read - only), WORM (write - once read - many) & EO (erasable optical disks). </Li> <Li> Magnetic Disk: A magnetic disk is a circular plate constructed of metal or plastic coated with magnetized material . Both sides of the disk are used and several disks may be stacked on one spindle with read / write heads available on each surface . Bits are stored on the magnetized surface in spots along concentric circles called tracks . Tracks are commonly divided into sections called sectors . Disks that are permanently attached and cannot be removed by the occasional user are called hard disks . A disk drive with removable disks is called a floppy disk drive . </Li> <Li> Magnetic tapes: A magnetic tape transport consists of electric, mechanical and electronic components to provide the parts and control mechanism for a magnetic tape unit . The tape itself is a strip of plastic coated with a magnetic recording medium . Bits are recorded as magnetic spots on tape along several tracks . Seven or nine bits are recorded to form a character together with a parity bit . Read / Write heads are mounted in each track so that data can be recorded and read as a sequence of characters . </Li>

What is the purpose of secondary storage device