How critical is the data on the drives? Personally, if a drive kicks out once, it was weird. If it happens again soon, it gets replaced. Soon is a relative term depending on the criticality of the data. If it's a production server at work, a drive getting dropped twice in six months is too often. At home, if it happens twice in a week, I may just backup more carefully for a while.

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And from what I've seen, that's too often for even at home. If it were me, I'd sleep much better if I had replaced it. It may never fail, but how much is your time worth if you had to restore from backups?