Question : Problem: System won't mount or recognize former external PATA drive internally attached.

I'm trying to frankenstein a couple of machines. I have several ATA and SCSIu3 drives that will work but I also have a stack of much larger capacity drives that have been removed from external enclosures and they are PATA. What's the difference? Do I need to get new controller cards? Why won't they show up on the ATA buses? Everything is jumpered correctly and the cables function fine. Yes, I've made sure the cables are 80-wire. I'm at a complete loss and need some explanation. Anyone have any ideas?

Answer : Problem: System won't mount or recognize former external PATA drive internally attached.

OK, rules that out.

I think we are down to two possible cause left:

1.  The firmware on the drives in question is not compatible (rare, but it happens)
2.  Format issue

What we ruled out:

1.  Drives are not too large - less than 128 GB
2.  You don't have a hardware issue - you have other drives that work.
3.  Drives are jumped correctly - Master on native ATA bus
4.  Disk Utility does not see the drives

Correct?

If the drives are formatted correctly (HFS+), jumped correctly (master) Installed correctly (data cable and power cable plugged in), Disk Utility should see it.  All that is left is a format issue or  a drive firmware issue, but how to test/change/verify if DU does not see it, and you don't have access to a PC to test with?

I don't think you specifically covered this, but are you sure the drive is powered correctly and spinning up?  You can (carefully!) start up the G4 with the case open and your fingers on the top drive, and you can feel/hear the drive spin up.  Might want to verify that if you haven't yet.