Choosing a boot drive?

I have a couple of computers at my work that have two drives in them, one main drive and another that is cloned to using casper. Periodically the computer will boot off of the clone drive, leaving you on what can be up to a week old file system. We have tried to specify drives in the bios, but it only gives the option for "Hard Drive", not individual ones. Is there any way to force it to boot from the correct one without bios?