Hi System, ok. So try this with your SCSI:
Go into MASTER, DISK UTILITIES(7), CHANGE SCSI ID(0) and make sure that your EIII is on ID 5 or 6.
Then in the same module screen (as above), select HD300 as the device to change ID#'s on and make sure that it is set to SCSI 3. Set your external drive to SCSI channel 3 and remember to save the system (just hit exit or enter).
Then power off EIII and external drive. Power on external drive FIRST and let it settle down. Then power on your EIII that I was going to buy;)
If the SCSI ID's are correct, when the EIII's system loads and it scans the SCSI bus, you should see the drive's HD led light up for a moment as the drive gets mounted.
If all was successful, you should now be able to hit LOAD BANK and see the external drive in there. The internal drive that came with your EIII is most likely on channel 1, so just make sure that nothing has the SAME channel as anything else.
I know this is confusing to start with, but if all else fails drop me a line cuz I have been dealing with SCSI drives/issues/setup a LOT as of late. A lot of the time, it is purely trial and error.
I hope this gets you started in the right direction!!!!
Jay