Seagate working now
Okay - after running a mostly symbolic check of the drive with Disk Utility, and resetting the PMU (unplugging all peripherals and power for 10 minutes) - the iMac will boot once again from the Seagate FreeAgent 1tb drive. So I guess you're right - it was the latter. Both the WD and OWC drives must not support Firewire booting on this particular machine. Bummer.