What does it do?
CD-ROM drives, and their close relatives DVD and CD-R drives, provide access to data on CD and similar media. Although they look similar, computer CD-ROMs, CD-R and audio CDs all use slightly different software formats, while DVD is different again. CD-ROMs are ‘read-only memory’ which can’t be written to. CD-R (recordable) allows a writer drive to ‘burn’ files onto recordable discs. CD-RW is a variant allowing limited re-writing. Audio CDs are similar to CD-ROM, but contain audio files which can be played in audio CD players. DVD is another family of disk types and formats. Capable of storing much more information on disks of the same size as CDs, they include video (as in DVD video players), ROM and other variants. How does it work?
Most drives have a built-in carrier tray, on which the CD or DVD is placed. When inserted, a central spindle lifts the CD clear of the tray and spins it at high speed. (iMacs now have a trayless mechanism.) The spiral track in which data is recorded is then read by a red laser bumps in the track contain the digital data. CD and DVD mechanisms are similar to those found in domestic players, but their supporting electronics are different. On recordable media the spiral data track is ‘burned’ using a more powerful laser. What can go wrong with it?
Most drives are mechanically very reliable now. Common problems (apart from a scratched disc) stem from dust and dirt on the lens of the laser. This afflicted early Apple and third-party drives badly, and they required regular cleaning. Use a good-quality CD lens cleaner, which are available from hi-fi suppliers. Mac OS support depends on software drivers, originally in Apple CD extensions but now in a combined CD/DVD driver. How can I upgrade it?
As they are modular in design, most internal drives can be upgraded but with limited benefit. Switching to DVD is attractive but more complex: to watch DVD video the output must be decoded, either using a special add-in card or, on the latest Power Macs, in software. Upgrades are best left to experts or the very adventurous. Further info
Web sites worth visiting include drivers for older models at members.xoom.com/macdrivers/disk.html, and an overview of DVD technology and formats at til.info.apple.com/techinfo.nsf/artnum/n24451. | 
