This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
SCSI ID numbers identify devices on the SCSI chain. Each device should have its own, unique ID number in the range 0-7. (If two devices on the SCSI chain have the same ID number, there will be a conflict and your system will not function correctly.) Higher numbered devices have higher priority-- get 'looked for' first-- so, it is standard practice to set the device you boot from to 6 or 7. Most external SCSI devices have a thumbwheel switch, slide switch, or jumper block on the back to set ID number. Some, like the Creative x2 CD-ROM drive let you click through 0-7. The Zip Drive lets you pick 5 or 6. (By the way, SCSI ID numbers have nothing to do with which Slot the SCSI interface card is in.) --------------------------- By: David Empson SCSI ID 7 is usually special because the Apple SCSI and Hi-Speed SCSI cards count as a device set to ID 7 by default (and every Macintosh has a hard- wired SCSI ID of 7). The only thing that is special about ID 0 is that it is the standard ID used for an internal drive on a Macintosh. There is no problem using SCSI ID 0 on an Apple II. On a RamFAST SCSI card, it is also safe to use SCSI ID 7 for a drive. The RamFAST doesn't have a SCSI ID, but every other SCSI card does. ___________________________ By: David Empson