A few months ago I had considerable trouble installing Hardy heron on a
computer with mixed drives.

I think that the problem was that Ubuntu now labels all the drives as
SATA, ie sda, sdb, sdc etc even if some are IDE. Older versions of
Ubuntu label IDE drives as hda, hdb, hdc etc. In a mixed drive system,
the first SATA drive gets labeld as sda.

By default Ubuntu will expect the hardware to boot from the first SATA
drive ie. sda. It's possible that your hardware may be set up to boot
from the first IDE drive, as mine was, which is why it didn't work.

I also solved the problem by physically disconnecting the SATA drive
during installation of Ubuntu (IIRC disabling the drive in the CMOS
didn't work).