Sounds like a SCSI termination or conflicting SCSI ID issue to me.

Try resetting all of the jumpers to their default positions.

Assuming your drive is some Apple variation of the Seagate Hawk 2XL Disc Drive you can download the product manual (with pictures of where to place the jumpers) here:

http://www.seagate.com/support/disc/manuals/scsi/77767489.pdf

If the default positions don't work, try changing the SCSI ID to "1" instead of "0"

If that doesn't work, try switching the position of the "Terminator Enable" jumper.

Hope this helps!