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just as long as internal, well if your not abusive. Expect beyond 5 years easy. If its fine for the first 3 months, it should be fine for a long time.
 
5 years

Put it this way--the Seagate external drive I have has a 5 year warranty. That should tell you how long it should last. Granted, not every part made will last to specs, but manufacturers always under warranty the parts' lifespan rather than over warrantying them.
 
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