I have a mixture of USB and FIrewire external drives and none of them have a problem booting.
Certainly your iMac will boot from Firewire or USB without a problem and this has been the case with Macs powered by the Intel processor since the first one.
All Macs have been able to boot from FIrewire since its introduction may years ago but USB booting will only work with Intel powered Macs.
Both Seagate and WD drives, at least all the ones that I have owned have been capable of Firewire booting and USB with the correct machine.
A couple of my drives were doing duty attached to a PPC mac and are now in USB enclosures and booting as expected.
Without teaching you to suck eggs, how do you have the jumpers set on the two drives?
I'm really frustrated here:
I'm using a mid-2007 20" iMac. I've always backed up using SuperDuper to clone to a Firewire hard drive. I seem to recall that for the first few years, I had no trouble booting from those backups. At the time, only the most recent machines could boot from USB.
I replaced my Seagate backup drive with a Western Digital drive a few years ago. That drive wouldn't boot via Firewire. It would only boot via USB - a limitation confirmed by WD.
Recently, I upgraded that drive to an OWC Mercury Elite. I'm having exactly the same problem. I pulled a Seagate FreeAgent drive out of mothballs and cloned to it, and though it won't boot fully either, it gets to the Apple logo before being rejected.
Am i mis-remembering booting this machine from Firewire? Is this just a limitation of newer drives? Did I just happen to purchase two drives from different manufacturers with hobbled Firewire?