Assuming the CD is SCSI, try "Command-Option-Shift-Delete" paired with the ID #. That will at least tell you definitively whether there's a problem with the CD's bootability (from the LCIII's vantage).
(Sort of off-topic but on Powerbooks, for instance, that use both IDE and SCSI I've never figured out how that works. I assume that command only affects the SCSI bus.)
(Sort of off-topic but on Powerbooks, for instance, that use both IDE and SCSI I've never figured out how that works. I assume that command only affects the SCSI bus.)