An actuator moves a laser beam spot to {TAIL ~ a desired track} of {HEAD ~ an optical disk} 
An actuator moves a laser beam spot to { TAIL ~ a desired track } of { HEAD ~ an optical disk }
OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD