Booting 10.5 froon G5 PPC from an externa

Ok here goes..
So I got myself an iPhone4 and was really excited to try it when i got home and instal it on my G5 tower PPC only to find I needed OSX 10.5.8 as a minimum requirement to run the iTunes installer.
Anyway I bought a second hand copy of Leopard 10.5 and began an instal.
Halfway through, the instal crashed and failed, leaving me with a computer and a half installed OS that won't boot! With hindsight I should have 'archived and installed' or installed to an external drive! 
To make matters worse I now don't have enough room to reinstall from the DVD even with all the unnecessary extras deselected.
So I now have a FireWire drive which I have successfully installed 10.5 onto and when I use startup disk it sees my FireWire drive with 10.5 on it but it won't boot off of it when I select and restart.
I have also tried restarting holding down the option key and the FireWire drive doesn't show up! Even though it does in disk utility.
I just want to boot from the FireWire 10.5 so I can use my machine again and my new iPhone4!!!
If anyone can help me in my sorry predicament i would be eternally grateful.
Kind regards


Hi Sebastian, and a warm welcome to the forums!
Some Firewire drives will not boot a PPC Mac, especially if it has a one button touch feature.
Like these models to name just a few...
The following external hard drives are not bootable on Power PC based Macintosh computers systems through USB/FireWire 1394(a/b):
• My Book Essential Edition
• My Book Essential Edition 2.0
• My Book Home Edition
• My Book Mirror Edition
• My Book Office Edition
• My Book Premium Edition II
• My Book Pro Edition II
• My Book Studio Edition
• My Book Studio Edition II
• My Passport Studio
• My Passport Elite
• My Passport Essential
• WD Passport
• WD Passport (Silver)