"Martin 53N 1W" <ml_news@ddnospamddml1dd.co.uk.dd> wrote:

Spin-up and spin-down, and thermal cycling of HDDs reduces their life
span. However, leaving them running continuously also causes them to
wear and fail. I've never seen any hard numbers to say to what extent or
for what significance each failure mechanism combines to cause a fail.

There was an extensive discussion of this back about 5 years' ago on
this group, entitled "Donate your hard disk to SETI@home."  The conclusion
was as follows:

Laptop disks:  the lifetime is *not* affected by frequent spinning up/down,
but running 24/7 is bad for the bearings.

Desktop disks:  running 24/7 is best; spinning up/down shortens lifespan.

The point being that laptop and desktop disks are very different beasts and
cannot be treated in the same way.