I believe (but can't cite any sources for that) that unless the head is parked (which either only happens when the disk goes to sleep, or at least after somewha

I believe (but can't cite any sources for that) that unless the head is parked (which either only happens when the disk goes to sleep, or at least after somewhat extended times of inactivity) HDD heads are always right above the platter, i.e. regardless of whether something is being read, written, the head moved, or there being a (short) activity pause. Assuming that the number of repolarisations of the magnetic substrate is not limiting, while total parking/sleep times should have a +-strong correlation with life times (although it might be negative, too, when the periods are too short, due to parking cost?), total bytes read/written should only have a weak correlation (by being correlated with the former). I am not a storage device specialist.