I had exactly the same problem with my HD. Bought a brand new 20 gig HD, installed and partitioned it (was shown quite nicely in HDToolBox) and rebooted...no HD seen anywhere. Tried just about anything to get it working without success. Then I decided to try it on the secondary bus as master (previously I had tried it as slave in the primary bus) and - it appeared on WB as if by magic. It could be that this HD of yours is like the one I have - It doesn't like being a slave (I'm quite sure I had jumpered it properly). If possible you could try setting it to be master on secondary bus, other than that I have no idea.
Oops, I just noticed it was a SCSI, not IDE. Still, it could be a problem with jumpering or your SCSI set-up.
J.