I know a great mechanic that can help you - he works in Aldershot - fantastic and truly honest. He could answer your question...because I can't. There is a whole magnet drive assembly in these compressors, odd though for the compressor to seize up when not on - yet turn freely when the AC is on. Almost sounds electrical more so than mechanical.... maybe the electrics were reconnected wrong...causing the compressor to try to spin in the opposite direction thus the grinding, because that compressor should freewheel if not in service. If it where mechanical, I would think the compressor would seize regardless if its in service (on) or not (off)
If you want the name of the mechanic just send me a note.
GO BABY GO! 
