I've never had a problem with any old internal 50 pin SCSI CD-ROMs I've found. I've used quite a few including several common Apple models with no problems.
As stated... Termination and Unit ID are important. Check out a SCSI tutorial somewhere. You need termination on both ends of the chain. If you had an external CD-ROM that supplied termination, you may have lost that termination when you unplugged it. It's not a given that a SCSI card will auto-terminate the external side of the chain when nothing is plugged into it.
Also, make sure you have no unit # conflicts. I tend to shy away from using unit ID 0 and 7, as it's easy to forget that the SCSI controller needs a unit number, too. ;-)
Dealing with the old SCSI-1 is one of those pain-in-the-ass things that I really don't miss...
