A 9500/150 here, a known good HD installed internally , the machine gets to the happy Mac and goes no further. The same drive in an external enclosure connected to the external SCSI bus, the machine boots right up. New mobo installed, nothing changes. With the HD in the external enclosure and the machine booted into 9.1, System Profiler says the HD id is 0, the CD id is 3, so it does not appear to be an id conflict.

Could this be as simple as a bad ribbon cable? I can't think of anything else after changing the mobo.