Types of Data Storage Disc

In computing, there are two major types of data storage system: optical discs and magnetic disks.

1. Optical Discs

An optical disc is a flat, typically circular, polycarbonate disc that contains data, video or audio encoded in tiny bumps (or pits) on one of its flat surfaces. The data can be accessed when the disc is illuminated with a laser diode, and is read in a spiral path extending from the innermost track to the outermost track. The track pitch (the distance from the centre of one track to the centre of the next) is typically 1.6 µm (microns).

An optical disc is designed to support one of three recording types:

• read-only (e.g. CD and CD-ROM)
• recordable (e.g. CD-R, which can be written only once)
• re-recordable (e.g. CD-RW, which is rewritable)

Optical discs are usually between 3 -2 inches (7.6-30 cm) in diameter, with 4.75 inches (12 cm) being the most common size. Although the side of the disc that contains the data is typically coated with a transparent lacquer, optical discs do not have an integrated protective casing and are therefore susceptible to data transfer problems due to fingerprints, scratches, and other environmental problems.

Types of Optical Storage Discs

Blu-ray Disc (BD)

A high-density optical disc designed mainly for high-definition video storage, and intended to supersede the standard DVD format. While a standard DVD uses a 650 nanometre red laser, Blu-ray uses a 405 nm blue-violet laser, which allows for almost 6 times more data storage than a DVD. The disc has the same physical dimensions as standard DVDs and CDs, but can hold up to 25 GB per single layered disc, and 50 GB per dual layered disc.

Compact Disc (CD)

A CD is an optical disc that was originally developed to store sound recordings, but later extended to preserve other types of data. Commercially available since October 1982, they still remain the standard physical storage medium for audio recordings.

Standard CDs can hold up to700 MB of data (80 minutes of uncompressed audio), whilst Mini CDs, often used for device drivers or CD singles can hold up to 210 MB of data, depending on their size. The smallest, the 60 mm disc (half the diameter of a standard disc), can hold up to 50 MB.

Digital Versatile Disc (DVD)

Also known as Digital Video Disc, the DVD is an optical disc storage media format designed for video and data storage. They have the same physical dimensions as a CD, but can store far for data. A single sided, single layer DVD can hold 4.7 GB of data, whilst a double-sided, single-layer DVD can hold 9.47 GB.

High-Definition/Density DVD (HD DVD)

This high-density optical disc was developed for the storage of data and high-definition video, and was intended to become the successor to the standard DVD format. HD DVD could store 15 GB per layer (compared to 4.7 GB per layer for a DVD). However, the format was abandoned in 2008 in favour of the Blu-ray Disc (BD).

Holographic Versatile Disc (HVD)

This next generation ultra-high-density optical disc format is designed to eventually become a replacement for the Blu-ray Disc (BD). HVD discs theoretically have the capacity to hold up to 6 terabytes (TB) of information, and a number manufacturers plan to release 1 TB capacity discs by 2016.

Laserdisc (LD)

The Laserdisc (LD) was the first commercial optical disc storage medium, originally marketed in 1978. The disc could hold up to 30 minutes standard play video on each side, and 60 minutes of extended play. Laserdiscs were popular in Japan, Hong Kong and South East Asia as an alternative to VHS tapes, but remained a relatively obscure format in Europe and North America.

MiniDisc (MD)

The MiniDisc, introduced by Sony in 1992 is a magneto-optical disc-based data storage device initially designed to store up to 80 minutes of audio. They were designed to replace cassette tapes, but only met with only limited success, faced with competition from the recordable compact disc. Another version of the disc, MD-Data was designed to hold up to 650 MB data, and a further version, the Hi-MD format, introduced in 2004, offers 1 GB of storage and the ability to act as a USB drive. Hi

2. Magnetic Disks

A magnetic disk stores information on round, glass or aluminium platters covered with a delicate magnetic material. The surface of the disk is shiny, like a mirror, and it is this polished surface that is critical to the proper functioning of the disk - even the smallest amount of dust can cause permanent damage. The data is stored on the magnetic platter and is read by a head (or more commonly, heads) on an arm that is attached next to the spinning platters. This arm can move across the surface of the platter as many as 50 times in a single second. The data isn't stored on the platter in a linear manner, but is simply held in the best way for quickest retrieval.

Types of Magnetic Storage Discs

Floppy Disk

A floppy disk is made using a circular piece of thin, flexible magnetic storage medium encased in a rectangular or square plastic shell. Invented by IBM, they became the standard form of data storage and exchange, from the mid-1970s to the late 1990s. The floppy disk case was inserted into a compatible disk drive, where the data could be read and/or written. There were a number of formats, ranging from an 8 inch version that held just 175 kB, to a 3.5 inch variety released in 1987, which could hold 1.44 MB of data. Floppy disks have now been largely replaced by external hard drives, USB flash drives, CDs, DVDs, and memory cards, although they may still be used with legacy industrial computer equipment.

Hard Disk Drive (HDD)

A hard disk drive is a non-volatile storage device that stores digitally encoded data on rapidly rotating platters with magnetic surfaces. The data is stored by magnetising ferromagnetic material directionally, to represent either a 0 or a 1 binary digit, and is then read back by detecting the magnetisation of the material. The disk is typically held in a rigid, sealed unit, which allows much tighter tolerances than in a floppy disk drive. This means that hard disk drives can access and transmit the data faster than floppy disk drives and can also store far more data (highest capacity is currently 2 TB). The most popular form factors for hard disk drives are 3.5" and 2.5", with platters rotating at speeds of 5,400 to 10,000 rpm.