I have a 3k with an external SCSI box. I don't have any problems.
You might try to get a internal to 25pin SCSI adapter and use it to connect the external SCSI cable to the internal one on your SCSI box. See if you have the same problems.
If I were home I would try to dig up my 3K schematics and look around the SCSI section to get an idea of what it could be.
Andrea
