Compact Disc ReWritable (CD-RW) is a rewritable optical disc format. Known as CD-Erasable (CD-E) during its development, CD-RW was introduced in 1997, and was preceded by the never officially released CD-MO in 1988.

   While a prerecorded compact disc has its information permanently written onto its polycarbonate surface, a CD-RW disc contains a phase-change alloy recording layer composed of a phase change material, most often AgInSbTe, an alloy of silver, indium, antimony and tellurium[1]. An infra-red laser beam is used to selectively heat and melt, at 400 degrees (Celsius), the crystallized recording layer into an amorphous state or to anneal it at a lower temperature back to its crystalline state. The different reflectance of the resulting areas make them appear like the pits and lands of a prerecorded CD.

   CD-RW discs are usually produced in the most common CD-R disc capacities such as 650 and 700 MB, while smaller and larger capacities are rarer. CD-RW recorders typically handle the most common capacities best. In theory a CD-RW disc can be written and erased roughly 1000 times[2], although in practice this number is much lower. CD-RW recorders can also read CD-R discs. When used with traditional recording software, 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 writing sessions must be closed before they can be read in CD-ROM drives or players. One important difference between R and RW media is that the re-crystallized alloy of RW media will gradually decrystallize over time. Because of this, CD-RWs are not as reliable for long-term storage; however, under recommended storage conditions, CD-RW should have a life expectancy of 25 years or more (as compared to 30+ years for CD-R).

Applications and limitations

   CD-RW discs never gained the widespread popularity of CD-R, partly due to their higher per-unit price, lower recording and reading speeds, and compatibility issues with CD reading units, as well as between CD-RW formats of different speeds specifications.

   Also, compared to other forms of rewritable media such as Zip drives, Iomega Jaz drives, Magneto-optical and flash memory based media, the CD-RW format uses the standard CD-ROM and CD-R file systems and storage strategies, which are inherently unsuitable for repeated small-scale file additions and deletions, thus making the use of CD-RW as a true removable disk impractical.

   CD-RW also have a shorter rewriting cycles life (ca. 1,000) compared to virtually all of the previously exposed types storage of media (typically well above 10,000 or even 100,000), something which however is less of a drawback considering that CD-RWs are usually written and erased in their totality, and not with repeated small scale changes, so normally wear leveling is not an issue.

   Their ideal usage field is in the creation of test disks, temporary short or mid-term backups, and in general, where an intermediate solution between online and offline storage schemes is required.