Compact Disc RewritableIn computing and data storage, Compact Disc Rewritable, or CD-RW, is a rewritable version of CD-ROM. A CD-RW drive can write about 650 megabytes of data to CD-RW media an unlimited number of times. Most CD-RW drives can also write once to CD-R media. Except for the ability to completely erase a disk, CD-RWs act very much like CD-Rs and are subject to the same restrictions; i.e., they can be extended, but not selectively overwritten, and must be closed before they can be read in a normal CD-ROM drive. A variation of UDF formatting allows CD-RWs to be randomly read and written, but limits the capacity to about 500MB.CD-RW media cannot be read by CD-ROM drives built prior to 1997 due to the reduced reflectivity (15% compared to 70%) of CD-RW media. CD-RW drives and media are more expensive than CD-R drives and media. CD-R is sometimes considered a better technology for archival purposes as the data cannot be accidentally modified or tampered with, and encourages better archival practices. Standard prerecorded compact discs have their information permanently stamped into an aluminium reflecting layer. CD-RW discs have a phase-change recording layer and an additional silver (aluminium) reflecting layer. A laser beam can melt crystals in the recording layer into a non-crystalline amorphous phase or anneal them slowly at a lower temperature back to the crystalline state. The different reflectance of the areas make them appear as the 'pits' and 'lands' of a standard CD. See also: computer storage, computer hardware, disk or disc, DVD Rewritable.
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