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