OK dude, look to see what SCSI ID # your SCSI card is using. For instance, on Adaptec cards the SCSI card is set to SCSI ID 7. So you can't use that ID for ANY device on that channel. If your card is using ID 1 and you set you HD to 1, you're going to have problems. If your card is using SCSI ID 7, set all other devices to anything but 7 and terminate the last device and you should be good to go. Read your manual to find out what SCSI ID your card defaults to. 😉
Also (I know you probably did this already), make sure the HD you are going to boot from is set to ID 0 and ONLY the last device on the channel terminated.