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How Computer Data is Stored: an Introduction

The goal of computer data storage is to provide a means to accurately store and access digital (binary) data. Storing data digitally provides a number of advantages, the most evident being resistance to corruption. This is due to the fact that each bit can be interpreted as being in one of only two states. Digital storage also enables error detection mechanisms which guard against data errors.

Magnetic storage
Magnetic storage disks are coated with tiny particles which are magnetically sensitive, allowing the read/write head of the storage device to change the magnetic alignment of the particles to represent binary data. The reverse, simply passing the head over the surface of the disk, allows it to read the magnetic field alignment and thus the data. Common magnetic devices are floppy disks, hard disks and magnetic tape.

Optical storage
Optical storage devices use a laser beam to read information from optical disks. The surface of the disk has spots that reflect light, which are interpreted as a binary '1', and spots that do not reflect light, interpreted as a binary '0'. Optical disks, unlike hard disks, are usually removable, which is a trade-off: access time and speed of data retrieval is sacrificed for the convenience of removable and portable media. Optical storage media includes CD-ROMs, DVD-ROMs and various formats of writeable and re-writeable disks, such as CD-R, CD-RW and DVD-R.

Magneto-optical storage
Magneto-optical storage uses both magnetic and optical technologies to produce disks with a large capacity in a portable format. The disks are coated with a thin, clear layer of plastic over a layer of magnetically sensitive particles. The plastic layer prevents the magnetic particles from being affected by the write head until a laser beam melts it, after which the plastic quickly cools and hardens again. To read the data a laser beam is focused on the spinning disk while a sensor detects changes in polarisation of reflected light from the surface.