A direct access storage device that is written and read by light. The most common optical discs in use are CDs and DVDs, and they come in three types: read-only, write-once and rewritable. CD, CD-ROM, DVD-ROM, DVD-Video as well as BD-ROM (Blu-ray) are read-only discs that are recorded at the time of manufacture and cannot be erased. CD-R, DVD-R, DVD+R, BD-R, WORM and magneto-optic (in WORM mode) discs are write-once. They are recorded in the user's environment, but cannot be erased. CD-RW, DVD-RAM, DVD-RW, DVD+RW, BD-RE and MO discs are rewritable.
As removable media, optical discs have advantages over the mostly obsolete removable magnetic disk cartridges. They have higher capacities and are not subject to head crashes or corruption from stray magnetic fields. They also have a 30-year life and are less vulnerable to extremes of hot and cold. However, external hard disks that plug into the USB port have given optical discs a run for their money as a transportable storage medium. They have higher capacities than CDs, DVDs and even Blu-ray media, and they are increasingly smaller and lighter.
Magneto-Optic (MO) and Phase Change
Rewritable discs use either magneto-optic (MO) or phase change technology. Used in libraries that hold multiple cartridges, magneto-optic (MO) discs are extremely robust. Phase change disks (CD-RW, DVD-RAM, etc.) are lower cost consumer-oriented products. See DVD, phase change disc, holographic storage, ISO 13346, multilevel optical disc and legality of optical storage. See also magnetic disk and magnetic tape.
Writability Optical Disc Types
Read only CD, CD-ROM, DVD-ROM,
DVD-Video, BD-ROM
Write once CR-R, DVD-R, DVD+R, BD-R, WORM
Rewritable CD-RW, DVD-RAM, DVD-RW, DVD+RW,
BD-RE, magneto-optic (MO)
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