USB HDD's would work well in this situation. Sure you could get a 2 or 3TB NAS and do the same thing, but then your putting all your egg's in one basket. If there's a fire your one and only NAS will probably burn with your server. At least if you have USB HDD's you could have one off site at all times. You need to be careful with transporting HDD's though, they are prone to failure if roughly handled. This is why it's usually preferable to use tape for offsite backups, they're a bit more resilient.