The write stuff Writeable CD technology has travelled a rather rocky road to its current position and one reason involves the fundamental differences between the way CD-ROMs and magnetic media operate. In contrast, audio CDs and CD-ROMs store data as a pattern of pits in the surface of the disk, which reflect laser light to a differing degree to the surrounding aluminium layer. These pits are formed by a stamping process and they cannot be altered once the CD has been manufactured. The first hurdle in the way of consumer CD mastering was the problem faced by manufacturers of finding a technology that was writeable but would produce a finished disk which looked like a pressed CD or CD-ROM to any standard player. The solution that was eventually adopted was to create blanks with a layer of dye that could be burnt away by a relatively high-powered writing laser.
Rewritable CDs are an even later innovation and use a polycrystalline data layer.
Writing to one of these disks involves selectively heating the surface by laser, which alters the data layer from an initially 'ordered' state into an 'amorphous' one that reflects light differently. Unlike a CD-R disk, this media can be 'blanked' by formatting it back to its original state or it can be simply overwritten with new information. However, there is a limit on how many times you can write to CD-RW media before it wears out and most disks on the market are guaranteed to last only a thousand write cycles. Most of the latest CD-ROM drives and CD players on the market have been designed to take account of this, but older models may not be able to read them. There's also another reason why CDs aren't as easy to create as simply writing files to your PC's hard disk.