Can't Boot from Freecom 500GB Hard Drive?

I'm having trouble making a bootable clone of my internal HD on a Freecom external hard drive.
I don't know whether the problem is with my system or the external drive, but having just upgraded to 10.5.8 I thought I might as well start here.

System details are:

Computer
17" iMac
PowerPC G4
1.25Ghz
512Mb RAM
80GB Internal HD
OS-X 10.5.8 Leopard

External Hard Drive
Freecom 500GB Firewire 400 External Hard Drive
Part No: 31251
Pre-formatted HFS+ - Mac OS Extended (journaled)
Apple Partition Map
Four Partitions
1) 80GB
2) 20GB
3) 140GB
4) 160GB

I want to get a bootable clone of my internal HD onto Partition 4 of the Freecom external HD but whatever I try, my computer won't boot reliably from the Freecom.
I've tried with Carbon Copy Cloner and with Apple's own Disk Utility Restore feature. Both times the software seemed happy enough that all was well, but still my clone won't boot. I've run a permissions repair on the clone but again, Disk Utility said all was well.

I specifically bought a Mac friendly Firewire 400 drive so that I could create and keep a bootable clone.

Anyone else had any problems with these drives or any ideas of how to fix this?

Any thoughts or advice gratefully received.