CD-RW
  CD-Read/Write - allows many (100 times) re-writing of data.

  True deletion of data.
    By rewriting whole disk.

  UDF allows for more selective re-writes and erases but requires extra
    overhead.

  Dye replaced by a reflective layer of alloy metal :
    antimony and other metals.  AgInSbTe

  This alloy has two states, crystalline and amorphous.

  Crystalline is reflective (lands) > 25%.

  Amorphous is non-reflective (pits) < 12%.

  The CD-RW is equipt lasers with three power levels capable of
    Reading

    Converting to Crystalline (heated but not melted)

    Converting to Amorphous  (melted and quickly cooled)

  DVD +/- RW  use GeSbTe, another mix with similar propertires.
    Responds quicker and uses less power for phase changes.
   (GeSbTe can be also used as a memory cell, 
      crystaline conductive, amorphous resistive)