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Secondary storage devices are generally separated into three types:

• magnetic storage devices, such as hard disk drives and tapes
• optical storage devices, such as CDs, DVDs and Blu-ray discs
• solid state storage devices, such as solid state drives (SSDs) and USB memory sticks

Magnetic devices

Magnetic devices, such as hard disk drives and tapes, 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. Data is accessed directly on a disk drive by knowing the location of the data.

Magnetic hard disk drives can be internal - a component within the computer - or external portable devices. Internal hard drives are used for everyday storage and external ones are suitable for backing up home computer systems.

Tapes are commonly used for permanent storage of very large amounts of data. The data is read or written using a tape drive. Data must be accessed sequentially (serially) on a tape. This makes it slower to access since each record has to be read one by one until the correct record is reached. Once the correct location is found the data can be downloaded at very fast rates.

Magnetic devices are low cost, high in capacity and durable. However, they are susceptible to damage if dropped. They are also vulnerable to magnetic fields - a strong magnet might erase the data the device holds.

Optical devices

Optical devices such as CDs, DVDs and Blu-ray discs use a laser to scan the surface of a spinning disc made from metal and plastic. The disc’s 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’s 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:

• Read only (ROM) media have data pre-written on them. The data cannot be overwritten. Music, films, software and games are often distributed this way.
• Recordable (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 be written to only once, but can be read many times. Copies of data are often made using these media.
• Rewritable (RW) media work in a similar way to R media, but can be written to more than once.

Optical storage devices such as CDs and DVDs are suitable for storing multimedia files. The storage type is added to the end of the device name to show what type of media it is. For example, a CD-R is a recordable CD, and a DVD-ROM is a read-only DVD.

Solid state devices

Solid state technology is used in storage media such as solid state drives (SSD) and USB flash drives. The technology is called solid state as it does not have any moving parts, unlike magnetic and optical devices. Solid state devices are popular because they use low power, have high access speed and are durable, making them portable. A USB flash drive is suitable for moving small files from work to home.