Hard drive lifetime: wear from spinning up or rebooting vs running

[David Liontooth <liontooth@cogweb.net>]

In designing an archival system, we're trying to find data on when it
pays to power or spin the drives down versus keeping them running. 

Is there a difference between spinning up the drives from sleep and from
a reboot? Leaving out the cost imposed on the (separate) operating
system drive.

Temperature obviously matters -- a linear approximation might look like
this,

     Lifetime = 60 - 12 [(t-40)/2.5]

where 60 is the average maximum lifetime, achieved at 40 degrees C and
below, and lifetime decreases by a year for every 2.5 degree rise in
temperature.  Does anyone have an actual formula?

To keep it simple, let's assume we keep temperature at or below what is
required to reach average maximum lifetime. What is the cost of spinning
up the drives in the currency of lifetime months?

My guess would be that the cost is tiny -- in the order of minutes.

Or are different components stressed in a running drive versus one that
is spinning up, so it's not possible to translate the cost of one into
the currency of the other?

Finally, is there passive decay of drive components in storage?

Dave