Debian boots on one Mac but not another

view story

http://unix.stackexchange.com – I'm trying to boot Debian from a USB Hard Drive (using rEFIt as a boot loader), and am successful on two newer-model MacBook Pros and a Hackintosh (no surprise there), but I'm not successful on several other Mac computers at my university. rEFIt loads just fine and sees my Debian partition, but when I try to boot it, it gives the error: no bootable device - insert boot disk and press any key I've read that this can be caused by several things, but I believe the fact that I can boot on some Macs eliminates most of these possibilities. I'm not terribly well-versed in bootloaders and how (HowTos)