A hard disk drive (HDD; also hard drive, hard disk, or disk drive)is a device for storing and retrieving digital information, primarily computer data. It consists of one or more rigid (hence “hard”) rapidly rotating discs (platters) coated with magnetic material, and with magnetic heads arranged to write data to the surfaces and read it from them.
Hard drives are classified as non-volatile, random access, digital, magnetic, data storage devices. Introduced by IBM in 1956, hard disk drives have been the dominant device for secondary storage of data in general purpose computer since the early 1960s.They have maintained this position because of advances which have resulted in increased recording capacity, reliability, and speed, as well as decreased cost, allowing them to keep pace with ever more demanding requirements for secondary storage.
A CD-ROM (an acronym of “Compact Disc Read-only memory”) is a pre-pressed compact disc that contains data accessible to, but not writable by, a computer for data storage and music playback.
CD-ROMs are popularly used to distribute computer software, including video games and multimedia applications, though any data can be stored (up to the capacity limit of a disc). Some CDs hold both computer data and audio with the latter capable of being played on a CD player, while data (such as software or digital video) is only usable on a computer (such as ISO 9660 format PC CD-ROMs). These are called enhanced CDs.
Blue Ray Disk
Blu-ray Disc (BD) is an optical disc storage medium designed to supersede the DVD format. The plastic disc is 120 mm in diameter and 1.2 mm thick, the same size as DVDs and CDs. Conventional (pre-BD-XL) Blu-ray Discs contain 25 GB per layer, with dual layer discs (50 GB) being the industry standard for feature-length video discs. Triple layer discs (100 GB) and quadruple layers (128 GB) are available for BD-XL re-writer drives.
The major application of Blu-ray Discs is as a medium for video material such as feature films. Besides the hardware specifications, Blu-ray Disc is associated with a set of multimedia formats. Generally these formats allow for the video and audio to be stored with greater definition than on DVD.
The name Blu-ray Disc refers to the blue laser used to read the disc, which allows information to be stored at a greater density than is possible with the longer-wavelength red laser used for DVDs.
A USB flash drive is a data storage device that includes flash memory with an integrated Universal Serial Bus (USB) interface. USB flash drives are typically removable and rewritable, and physically much smaller than a floppy disk. Most weigh less than 30 g.As of September 2011 drives of 256 gigabytes (GB) are available.
USB flash drives are often used for the same purposes for which floppy disks or CD-ROMs were used. They are smaller, faster, have thousands of times more capacity, and are more durable and reliable because they have no moving parts.