CD-RW and CD-MRW drives are dual-purpose devices. When writing CD-R discs, they work just like ordinary CD-R drives. But if instead you use CD-RW discs and the proper software, a CD-(M)RW drive can erase an old file and write a new file in its place.
A CD-RW drive works almost like an enormous floppy drive. Almost because unlike the floppy drive, the CD-RW drive requires special software to provide drive-letter access (DLA). A CD-MRW drive works exactly like an enormous floppy drive. Exactly because the Mount Rainier specification defines the features necessary in the BIOS, operating system, and drive to allow the PC to recognize the CD-MRW drive natively and assign a drive letter to it in the same way that it assigns a drive letter to a floppy drive or hard drive.
CD-RW is an extension of CD-R technology, initially championed by Mitsubishi Chemical, a major maker of CD-R media, and a group of CD-R drive manufacturers including Hewlett-Packard, Sony, Philips, and Ricoh. CD-RW drives and discs started shipping in mid-1997, just as CD-R seemed poised to become a mainstream technology. Rewritability was considered such a huge advantage that for a time it appeared that CD-R would disappear, killed by CD-RW. That turned out not to be the case. Relative to CD-R, CD-RW had several problems initially, most of which are no longer an issue with the latest drives, media, and software:
When CD-RW drives began shipping, they sold at a substantial premium—$250 ...