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Secondary storage devices are generally separated into three types:

• magnetic storage devices, such as hard disk drives
• optical storage devices, such as CD, DVD and Blu-ray discs
• solid state storage devices, such as solid state drives and USB memory sticks

Magnetic devices

Magnetic devices such as hard disk drives use magnetic fields to magnetise tiny individual sections of a metal spinning disk. Each tiny section represents one bit. A magnetised section represents a binary '1' and a demagnetised section represents a binary '0'. These sections are so tiny that disks can contain terabytes (TB) of data.

As the disk is spinning, a read/write head moves across its surface. To write data, the head magnetises or demagnetises a section of the disk that is spinning under it. To read data, the head makes a note of whether the section is magnetised or not.

Magnetic devices are fairly cheap, high in capacity and durable. However, they are susceptible to damage if dropped. They are also vulnerable to magnetic fields. A strong magnet might possibly erase the data the device holds.

Optical devices

Optical devices use a laser to scan the surface of a spinning disc made from metal and plastic. The disc surface is divided into tracks, with each track containing many flat areas and hollows. The flat areas are known as lands and the hollows as pits.

When the laser shines on the disc surface, lands reflect the light back, whereas pits scatter the laser beam. A sensor looks for the reflected light. Reflected light (lands) represents a binary '1', and no reflection (pits) represents a binary '0'.

Optical media also come in different types:

• ROM media have data pre-written on them. The data cannot be overwritten. Music, films, software and games are often distributed this way.
• R media are blank. An optical device writes data to them by shining a laser onto the disc. The laser burns pits to represent '0's. The media can only be written to once, but read many times. Copies of data are often made using these media.
• RW works in a similar way to R, except that the disc can be written to more than once.

Solid state devices

Solid state devices use non-volatile random access memory (RAM) to store data indefinitely. They tend to have much faster access times than other types of device and, because they have no moving parts, are more durable.

Since this type of memory is expensive, solid state devices tend to be smaller in capacity than other types. For example, a solid-state drive that holds 256 GB might be of a similar cost to a hard disk with several terabytes capacity.

Solid state devices require little power, making them ideal for portable devices where battery life is a big consideration. They are also portable due to their small size and durability.