Magnetic Storage Devices

Magnetic storage devices such as disks and tapes rely on a thin coating of a magnetic material on a plastic, metal or glass surface. Binary data are stored in the coating as small magnetic fields (either north or south magnetic poles), placed there by an electromagnet. The direction of the magnetic field (north or south) is used to represent the binary 0s and 1s. Reading the data stored in the coating involves detecting magnetic fields.
Magnetic tape is still commonly used for storing backup data, although writeable CDs and high capacity disks are becoming more popular. The tapes main advantages are its portability and low cost for the amount of data they can store. Most tapes are slow so they're only practical for long term data storage and backup. Data needed quickly are best stored on disks.

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Disks
Data are stored on the surface of a disk in tracks and sectors. The tracks are circles and the sectors are arc shaped sections. The number of tracks on a disk will depend on the disk size, type and storage capacity.