I got the same one Dave posted for my son's xbox and it's the best thing I ever bought.It sounds big but don't forget the games use lots of memory now.I've used 6tb already so I would get at least a 4tb drive minimum tbh.
Thinking of going for a 1TB baricuda drive at £40.. and the caddy for £5. Not a massive outlay and triple his memory. He doesn't alternate games all that often it's just the odd time he goes back to one he has had a while and has to delete one and reinstall and when he's done had to do the same.. I watched him do it the other day when he wanted to playpme FIFA and felt his pain lol.
The caddy idea seems a good idea till you realise they bumped the prices up on the 2.5" drives lol. So much of a muchness between one of these and an external, unless the transfer speeds are greater I Dono if I see much of an advantage