A CDE Definition
A direct access storage device that is written and read by light. The most types are CDs, DVDs and Blu-rays. As removable media, optical discs superseded the earlier magnetic disk cartridges. Optical media weigh less, 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 extreme hot or cold. However, as a transportable storage medium, USB-based flash drives and external hard drives are also widely used. Following are the major categories of optical discs based on writability. See CD, DVD and Blu-ray.
Read-Only (Factory Pressed)
Read-only discs are pressed from a master at the time of manufacture and cannot be erased. They include the music CD, CD-ROM, DVD-ROM, DVD-Video and BD-ROM (Blu-ray).
Write-once discs are recorded in the user's environment but cannot be erased. They include the CD-R, DVD-R, DVD+R, BD-R (Blu-ray), WORM discs, as well as magneto-optic (MO) discs in WORM mode.
Rewritable (Phase Change and Magneto Optic)
Rewritable discs can be written and re-written numerous times. Employing phase change technology, consumer-oriented products include CD-RW, DVD-RAM, DVD-RW, DVD+RW and BD-RE (Blu-ray) (see phase change disc). Magneto-optic (MO) disks combine optical with magnetic technologies (see magneto-optic disk). See ISO/IEC 13346, multilevel optical disc, holographic storage, WORM and legality of optical storage.
Writability Optical Disc Types
Read only CD, CD-ROM
Write once CR-R
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