The longer a drive is on, the warmer it gets
(at least within the first approx. 30 minutes).
The warmer it gets, thermic effects affect the
drive's mechanic. This causes it to expand over
time (and "shrink" if it gets cooler again).
So the position within the range where the
grove may exists moves.
This information is dated; modern hard drives store position info on the platter surfaces. The platters expand with temperature, but the feedback information for the voice coil expands right along with them, and the track should be written at the same position every time.
There is another effect that will cause the writing position to vary though. Hard drives will start writing before the process of zeroing-in on a track has completed, so some data will be written a bit off center as the vibration in the arm dies down.
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This information is dated; modern hard drives store position info on the platter surfaces. The platters expand with temperature, but the feedback information for the voice coil expands right along with them, and the track should be written at the same position every time.
There is another effect that will cause the writing position to vary though. Hard drives will start writing before the process of zeroing-in on a track has completed, so some data will be written a bit off center as the vibration in the arm dies down.
There's more here.