This is a known issue, which MOTU blames on Apple's USB implementation. It most commonly occurs when a MOTU USB interface is connected to the G4's USB port and powered OFF.

Solution : keep the MOTU USB midi interface _ON_ while shutting down the G4

* Some users have reported that the problem didn't occur if the MOTU USB interface was off, but connected to the Mac USB keyboard, or to a powered USB hub.