Usually, the firmware on a PPC does not lend well to booting from USB. You'll just have to try it. Some users have reported being able to boot their PPC machine from USB, others have stated they could not. Your best bet is to use a firewire external device to boot from. That works well.
Further, have tried literally dozens of G4 and G5 PowerMacs and yet to have one boot from an external USB drive. They will all, however, boot from an external Firewire drive, FW400 or FW800.
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