[dnanian]

It almost seems like the drive prompt, which would come up early, may not work with the different video card, since it may be done by part of the OS that doesn't support the card.

Since it's quite unlikely you'll ever boot from this drive (since any new Mac you get won't support the old OS), it may not matter that you can't boot from the backup. But, perhaps keep it unencrypted if that's proving problematic?