I had a similar problem with my MPC and CDROMs.
I had a CDROM(brand?) that i got from a friend & it worked fine at first. It later stopped working(being seen as a connected device), for some reason. I tried all SCSI adresses to no avail.
I then bought a highspeed multi media CDROM for MAC, which worked fine. About $200.00
I'm still not sure what happened.