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Physical Structure

A floppy disk is basically a circular sheet of plastic, coated with magnetic material. A hard disk is made of a stack of circular metal platters, also coated with magnetic material. Before a disk can be used it must be formatted. The surface of the disk is divided up into a number of concentric tracks, each of which is subdivided into sectors.

Floppy disks have 80 tracks on each side and each track is split into 18 sectors. A 3.5" floppy disk with 80 tracks and 18 sectors will have 80 x 18 = 1,440 storage units, each uniquely identified by its track and sector position. Each storage unit can hold 512 bytes of data, so the disk has a capacity of 1,440 x 512 = 737,280 bytes (720 KBytes) per side, or 1,400 KBytes (1.4 MBytes) per disk.

A hard disk is a sealed unit containing a stack of circular platters mounted on a common spindle. Electromagnetic read/write heads are located above and below each platter. The platters rotate at a constant speed, eg: 7200 rpm. While they are spinning the heads can move in towards the centre or out towards the edge. This allows them to reach any location on the platter.

Next: Hard Disk Structure