Disk model controls whether one can control boot order?

I've several systems built around:


WIth Western Digital drives - WDC WD800JD-00MS - the BIOS lets me choose
from which disk booting is to occur. With the Seagate equivalent, the
BIOS only lists one drive on that "page".

It really does appear to be a matter defined by the drives. If I open a
"list only one drive" machine and replace its two Seagate drives with
Western Digital drives, it becomes a "list two drives" machine.

I cannot imagine how the drives would be causing this in the BIOS. Any