I found I had to have the units plugged into the SCSI connectors closest to the SCSI card - if I put them further down the cable (for convenience) it wouldn't register them at all.
Also set the terminator on the last device connected, and made sure each device had an individual unit number selected (between #0 - #6, leaving #7 free, as this in the unit of the scsi bus).
Hope this helps.
Was just about to link to the same page as patrik before noticing the URL :-D
