R/W may refer to any of the following:
1. Short for Read/Write, R/W is a file attribute or permission that can be given to files and directories that allows them to be read or written. These attributes can also be taken away to prevent that file from being read or modified.
2. Short for Read/Write, R/W is a drive and CD media that was first introduced in 1997 that is capable of being written to and read. Unlike a traditional CD-R disc that can only be written to once, these discs allow data to be erased and re-written multiple times.
The CD-R technology uses a photosensitive dye, CD-RW discs use an active layer of Ag-In-Sb-Te (silver-indium-antimony-tellurium) alloy that, in its original state, has a polycrystalline structure that makes it reflective. When the CD-RW drive writes to the disc, the laser uses its highest power setting known as Pwrite. At this temperature, which is usually between 500 and 700 degrees Celsius, the chemical structure will liquefy. In its liquid state, the molecules of the active material flow freely, losing their polycrystalline structure and taking on an amorphous state. When the material solidifies in this amorphous state, it loses its reflectivity. By selectively firing the laser, the drive leaves parts of the disc in its polycrystalline state, forming the lands, and parts in the amorphous state forming the pits.
To reverse the phase of a specific area on a disc, the laser operates at a lower power setting and heats the active material to approximately 200 degrees Celsius. By heating the disc it reverts back from its amorphous to its polycrystalline state and then becomes reflective again.
The drawback with CD-RW discs is with the lower reflectivity of the disc itself can limit the readability. In the 1980s, the CD standards specified that on a compact disc the lands should have a minimum of 70% and the pits should have a reflectance of 28%. However, on a CD-RW disc, the reflectance of a land is approximately 15% to 25%. The low reflectance can cause certain CD-RWs discs to be unreadable in some older CD-ROM drives and CD players.