Is turning off hard disks harmful?

Solution 1:

Google showed that heat has very little effect on hard drive life expectancy, despite anecdotal claims to the contrary. See this paper. The paper was by far the largest study on the subject. Note, though, that Google keeps their drives running 24/7, so there was no information on specifically whether turning off your hard drives was harmful, only that you don't need to worry so much about temperature.

Solution 2:

Spinning down hard disks does help in reducing overall heat and also increasing its life.

However, there are reports that a lot of HDD failures happen when restarting a disk after the spindown.

This ServerFault answer to
What’s the effect of standby (spindown) mode on modern hard drives?
talks about it

More opinions on this are welcome.
I am also investigating the stability of using an older laptop with an alternate USB boot.