Gene Heskett composed on 2017-02-06 12:28 (UTC-0500):
That cold spare will eventually develop stiction, seizing the parked haed to the surface of the disk solidly enough that the disk motor cannot break it loose to spin the disk up. Such is best treated by hooking up the cables, but holding the drive in your hand so that you can turn on the power, and within a couple seconds, give the drive a good sideways blow on a corner with the ball of the wrist so the drive housing/casting is caused to rotate a few degrees around the axis of the disk, breaking the stiction so the spindle motor can spin it up. The theory is that the drive frame rotates when you drive it by hitting the corner, but the disk doesn't, breaking the stiction seal.
The other theory is the motor got too weak to start the platters in motion. It wouldn't surprise me that these things have separate electronics for startup and for maintain, and that something in startup simply expires to cause spinup failure.
The first HD I ever bought, a 3.5" "full-height" SCSI-I 80MB Seagate in 1990, acquired the won't spin up problem a month after its 12 month warranty expired. I avoided the problem a long time via a UPS, but eventually an extended power outage claimed it permanently.
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