I read once that the majority of drive failure happens during transitions to or from a running state, and thus it is counter-intuitively safer to just leave them running all the time.
I am sure there is some mathematical formula that will express the risk based on how infrequently one expects to spin the drive up (and then down again), but I don't know it.
Backblaze is hosted out of a SF datacenter, where power is much more expensive than other US locations; I assume someone has done the math whether its cheaper to replace failed drives from frequent spinups/downs vs burning the power to keep them spinning.
I am sure there is some mathematical formula that will express the risk based on how infrequently one expects to spin the drive up (and then down again), but I don't know it.