Comment 77 for bug 952556

The average laptop HD life can typically stand :

~ 40,000 - 60,000 disk spin-up

~ 600,000 - 800,000 heads load-unload cycles

So it's quite normal (and desirable) that you load-unload heads much more frequently than you spin the disk.

Furthermore loading heads is a matter of a fraction of a second, while spinning disk takes several seconds...

Furthermore an HD with "parked" heads is much much more shock-proof than an HD with flying heads (some laptops HDs have a freefall sensor that automagically parks the heads when the disk is falling and before it crashes on the floor, but this is outside the scope of this bug ;-) so keeping the heads parked as much as possible makes sense, when talking about laptops that are subject to shocks and vibrations.

So basically you need to divide the time you'd like your disk to live by 600,000 to determine the pace at which you would allow your heads to park, and by 40,000 to determine how often you'd like to allow your disk to spin down. Then take some safety margin ;-)