This item is from the Yet Another Enhanced IDE/Fast-ATA/ATA-2 FAQ, by John Wehman and Peter den Haan with numerous contributions by others. (v1.92).
o A single device on a cable, either a harddisk (ATA) or CD-ROM (ATAPI), should according to the specs never be configured as slave. However, as any ATAPI CD-ROMs come preconfigured as slave, most software works with a masterless slave CD-ROM simply because it saves tech support calls. Moreover some BIOSes have trouble with an ATAPI device jumpered as master. Formally it isn't a valid configuration though; remember this if you have trouble getting the CD-ROM recognized.
o Two harddrives on one channel should be configured as master and slave, respectively. It usually doesn't matter which is which, but older harddisks may not work in all configurations if they predate the standardization of the master/slave protocol. In that case a degree of experimentation will be necessary. Some Conner drives have an ATA/ISA jumper: ATA is the now-standard protocol, ISA is used by older Conners.
o Two ATAPI devices such as CD-ROMs and tapes should work fine as long as one device is master and the other slave, just like harddrives.
o If you need to put a harddisk and an ATAPI device on the same cable, the spec says the harddisk must be the master. Although the reverse will usually work as well in practice, still you'd better avoid the configuration.