I remember hooking up a 6500 to a 6100 one time and it worked - the 6100's hard drive showed up on the 6500.

Since the 6500 doesn't use a SCSI hard disk, it had nothing on ID 0. I turned on the 6500 first, waited a few seconds, then turned on the 6100, which was headless.

I guess what happened is that the 6500 got control of ID 7 first, and when the 6100 tried to take it, it failed and crashed silently, but allowed the internal SCSI hard drive to pass through.