Tim:
You might want to do what the message said, go into your BIOS, and play with the options for selecting which is your boot drive.
Excalibur Xcalibur:
Well this one doesn't work so well. The problem is that in the BIOS, both are *identical*. Meaning they have the same reference caled ST57XXX. So if I select this one, it doesn't boot the primary drive.
Seeing as you've got it working, you probably don't want to pursue the BIOS thing, but it shouldn't matter if you have two identical model drives (they're just names in a list, for your convenience), it cares about where they're connected. I've certainly dealt with this issue on one PC, here. BIOSs usually give you two ways to select the drive to boot from; picking from a list, or moving entries up and down in a list. Whilst it'd be hard to tell if any changes happened if they don't have different names, it should be possible to re-order things.