I am assuming that when you clicketh upon System Preferences in your MacPro . . . then go to "Start-Up Disk" it does show the Ex-HD as a legitimate start-up disk?
You may also be having a problem with the G4 being a PowerPC and the MacPro being Intel.
One other thing to check: make sure the Ex-HD is journaled. I wasted a lot of time trying to get my MacBook to recognized the Ex-HD as "bootable" until I realized I had not actually enabled journaling--some programs ran problematic off the Ex-HD as well until I "fixed" that. That may not at all be relevant to your situation, of course.
You may wish to post the full specs of your G4 and MacPro for the Gurus to chew over.
MacBookPro 2.8 GHz Intel Core i7, 16 Gig RAM, 10.8.3--"Okay . . . I will try Mountain Lion!"
Fear Me! FEAR ME!
His secrets are not sold cheaply.
It is perilous to waste his time.