Since you have tried two new drives I strongly suspect the problem is with their motherboard. The fact that the BIOS often fails to detect it is a very strong clue that it has nothing to do with Vista or with software. I would push for them to fix or replace.
Short of sending it back All I could suggest is trying the HD with a different cable and on a different SATA port. Since the secondary drive always seems to get detected try using your main drive on that cable and on that port.