Without additional s/w, you can always hold the 2 mouse buttons while booting, to get into the early boot menu. I think anything above the Ami 3000 has that, though I could be wrong. If you have multiple drives/partitions that auto-mount, you should be able to select which one you need that way. Otherwise, a script in the bootable partitions startup-sequence could remap the system assigns to another drive and hand off to its startup-sequence, though that can be a pain to set up. At least you wouldn't have to actually set another drive as a boot device.
EDIT: oops... Looks like you've aready been doing the early-boot thing. But the script might still work. leander's idea with linux might work, too.
