Could you have setup FileVault (or other HD encryption software) on your G4 systems HD's? If you have your external might be encrypted which would explain the odd reporting as the low level system tools see it yet the OS doesn't as it can't access it.
Can you setup both systems on a network with the external hanging off of the G4. Then see if you can access the G4 from the iMac over the network. If you can you should be able to copy over the files you need. If you can move everything over you could try reformatting the HD on the G4 (don't setup any partitions) then put the HD on the iMac and finish prepping it there and copy back the files. At that point you should be able to access the HD from either system.