in my experience hdds have signs they're going to die soon, they go slow, click a lot, get smart errors, things get corrupt etc. where as with flash media it all seems fine, but suddenly isn't.

not that this is a rule, i've seen hdds fail instantly for no reason as well, and flash media throw a massive wobbler but still be recoverable. but in my experience this is not the normal behaviour.

but who cares as long as you have backups and a contingency plan?