Earlier today, a "confirmed Xbox One developer" posted an image teasing the external hard drive. As you can see the Xbox One is recognizing the hard drive which offers over 600GB of space. And with the size of today's game installs, you're probably going to need it.
The developer didn't say much about the picture, but did leave an ominous "soon." It follows Microsoft's Major Nelson when he took to Reddit last month to also say that "External game storage is coming soon."
Microsoft has been pretty good at pushing out monthly updates for the Xbox One, each adding more user-requested features. With E3 right around the corner, I'm hoping we hear more about external hard drive support if we don't already have it by then.