Which reminds me . . .
Sometimes it isn't the drive itself that fails but the HD controller, the other electronics, etc. Problem can be solved by removing the drive and getting a new enclosure for it. And there goes a good chunk of the money you sunk into the unit as a whole. Sometimes it's a crap-shoot.
So know what kind of drive you're getting. Know what the enclosure is like - durable? Heat dissipation? Fan (when it *should* ideally have one)?
You can have a top-end drive in a lousy enclosure and it won't last.
You need to figure worst-case scenario here: if you've determined that it's a quality drive in a sturdy, supportive and properly-ventilated (or heat dissipating) enclosure, then your only worry (all else being equal) is the other electronics inside that control the drive. This is probably your best position, as the likelihood of you losing your data is much lower (ideally, at least.)
In any case, if you've got the money, opting for G-Tech drives will stack the deck a little more in your favour. Great drives (some of them Hitachi enterprise class) with well-eingineered enclosures.