Diskette Technology

Diskettes are 3.5-inch plastic squares that contain platters composed of a substrate and magnetic medium. The substrate (base material) must be non-magnetic and capable of being machined to a smooth finish. It is made of either a mixture of glass and ceramic or an aluminum alloy. Both sides of each platter are coated with a magnetic medium (a layer of metal called thin-film medium). This stores data in magnetic patterns, with each platter being capable of storing billions of bits per squares inch of platter surface.

Typically two or three platters (or more) are stacked on top of each other with a common spindle that turns the whole assembly at several thousand revolutions per minute. There is a gap between the platters making room for a magnetic read/write head, mounted at the end of an actuator arm. The platters are so close together that the magnetic head is only fractions of a millimeter above the disc.

The magnetic surface of the disc is treated as an array of dot positions. The position of each array element is not identifiable in an absolute sense, and so a scheme of guidance marks helps the read/write head find positions on the disc. This is why discs must be formatted before writing data onto them. Most diskettes store about 1.44 MB of data.