Auxiliary storage devices are also referred to as auxiliary memory or secondary memory. These devices supplement the main storage. Auxiliary storage devices are long term, non-volatile memory. Unlike RAM, it allows the computer to record information in semi-permanent way. Due to this reason, it can be read later by the same computer or by another computer too. These devices also function as backup devices which allow backup of the valuable data and information that the user is working on. So, even if by some accident, a computer crashes and the data in it become unrecoverable, the data can be restored from the backups. The most common examples of auxiliary storage devices are magnetic tapes, magnetic disks, Compact Disks (CDs), Digital Versatile Disks (DVDs), floppy disks, hard disks etc.
Auxiliary storage devices can be classified on the basis of two ways. They are:
1. On the basis of material used in the device: In this classification, the auxiliary storage devices are classified according to the material used in the manufacturing of the device. On the basis of material used, the devices can be classified into three categories which are mentioned and described below:
- Magnetic Devices: Magnetic devices contain magnets and magnetic substances. In magnetic storage media, the data and information are stored by polarizing the medium’s magnetic domains either in North Pole or in South Pole which represents the binary states, i.e. 0’s and 1’s. A magnet sensitive read-write head helps in the interpretation of these codes. Some examples of magnetic devices are hard disk, floppy disk, magnetic tape etc.
- Optical Devices: Optical devices are the medium for which data is read and written with the help of laser beams. Optical disks can store much more data than magnetic devices. Some examples of optical devices are Compact Disks (CDs), Digital Versatile Disks (DVDs) etc.
- Magneto Optical Devices: Magneto Optical Devices are the combination of the basic principles of both magnetic as well as optical devices. The Magneto Optical systems write magnetically with the help of thermal assist and read optically. At present, there are only two standard form-factors used for Magneto Optical systems. They are of 5.5 inch and 3.5 inch, which are protected by hard envelops. The larger form-factor Magneto Optical disks are capable of holding about as much as the standard CD-ROM. Under pressure from the cheap and relatively fast CD-R and CD-RW, Magneto Optical devices seem to be losing the ground. On the other hand, some of the principles of the Magneto Optical technology (thermally assisted magnetic recording) may fine their way into the most advanced magnetic storage devices of the future.
2. On the basis of the mode of data access: On the basis of mode of data access, the auxiliary storage devices can be classified under two headings. They are:
- Sequential Access Media: When we talk about the sequential access media, we can find that the data stored in the media can only be read in sequence. In order to get to a particular point on the media, the user must go through all the preceding (intermediate) points. Some examples of this media are magnetic tape, DAT cartridge etc.
- Random Access Media: Random Access media can access any point at random without passing through intervening points. Random access media is also known as direct access media. Some examples of random access media are hard disk, optical disks etc.