I'm assuming you're talking about an external hard drive here, since no computer has come without an internal one for a couple of decades now.
If it has moving parts or uses electricity, it can break--that's the rule of anything constructed my human beings that I'm aware of. Hard drives of course fit into both categories.
They're relatively reliable these days, but they do go occasionally (and eventually, if you use one long enough). Could be tomorrow, could be ten years from now. You might get warnings (funny sound, errors), or it could just go poof one day--I've had both happen to me.
That said, if you're using it for BACKUP, I don't so much see the issue--that is, if you've got one copy on your internal drive and a second on the external, then as long as they don't both die at the exact same time (which is theoretically possible, but statistically unlikely), then you're in good shape.
If you're only storing files on one drive or the other, then of course it's not backup, it's just storage. As with any computer file, it comes down to how important it is; if you could live with the loss of it, perhaps it's ok to have it on only one drive with no backup. If you don't want to chance losing it, have it in two locations. And if it's REALLY irreplaceable, put it on three, one of which is preferably optical media (CDR or DVDR), to hedge your bets against a lightning strike or something.
