I just picked up this 1tb drive from Fry's a couple weeks ago, works perfectly except for this bizarre double click noises that occurs probably once or twice every half hour. What's this a sign of?
It's probably just the drive re-calibrating the head servo.
As the temperature of a drive changes, the platters (and case etc) expand and contract, if the drive didn't re-calibrate the head positioning every now and then it could cause tracking errors; what you hear is probably the heads seeking to the inner and outer tracks.
Some drives will recalibrate as the temperature changes, some will do it when tracking errors are detected, some will just do it now and then when idle.
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