The first 1TB WD drive died this week. I had initially used it to back up all computers in the house. Then when one of the computers actually crashed, I used the WD for about 1 week before it died. But of course the whole even was so long after I originally bought it, I had somehow lost the receipt. (I do have the receipts for the WD Passport and WD MyBook, which is probably why they continue to run fine with daily use.
In short: An external hard disk will die when its warranty either expires or is lost, regardless of the amount of actual use.
(It may also die, even if under warranty, if it is the sole repository of some important file. In that case the disk will be replaced but the file will be lost forever.)