All three drives (a Seagate FreeAgent 1gb, Western Digital MyBook 2tb, and OWC Mercury Elite 3tb) are external drives. I'd have to open the enclosures (and void warranties) to check on jumpers.
I know that WD claims their drive won't boot via Firewire. The booklet that came with my OWC says this model of iMac is incapable of booting from Firewire (!). Not sure why the Seagate won't boot.