MTBF is based on statistical observations and projections from fairly large
samples of drives.  A common misunderstanding is that a drive with 350,000
hours MTBF will last for almost 40 years.  What the statistic is really
telling you is that, based upon normal usage, which includes replacement at
the end of the drives useful life, you should only have an abnormal failure
once every 350,000 hours.

Hopefully that makes sense.

Drew Dunn
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> This is more of a curiosity question than anything. I got to thinking the
> other day, how do they figure out the Mean Time Before Failure of a
> harddrive. If you break out your calculator, most major manufactureres are
> 30 years and upwards! For example, Western Digital lists a lot of their
> drives as 350,000 hourse(350,000 hours/24 hours in a day/365 days
> in a year
> = 39.95 years!!). I REALLY don't see this as at all possible for
> actual use.
> Anyone have any little insights on this?