A spindle motor operates dynamically in one of {TAIL ~ two modes} for {HEAD ~ a disc drive} 
A spindle motor operates dynamically in one of { TAIL ~ two modes } for { HEAD ~ a disc drive }
OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD