We got this question today: https://serverfault.com/questions/388656/hard-disk-lifespan
I voted to close it as a duplicate of this one: Does an unplugged hard drive used for data archival deteriorate? I think this one has pretty good answers, even if many of them amount to "I've had and everyone I know has had an old drive refuse to spin up." At one point I did some research to see if there were any hard data, but didn't find any (but I did learn a bunch about the GMR effect.)
There's also this old question: Is it safe to store data on a hard drive for a long time? But I don't think the answers are as good as on the previous question. (e.g. squillman's is wrong for disks that use GMR)
I just wanted to get agreement on what to do with this type of question. I'd say "close as dup of the one I quoted, maybe lock that one (although it doesn't really garner any "me, too" posts) and allow it to accumulate any new answers if anyone does come up with studies of how likely - or not - stored hard drives are to be dead.