A confirmed Xbox One Dev on reddit, posted this snapshot earlier today stating, “I give you this. Soon…”:
As you can see, the external hard drive notification popped up stating that it was ready for use. An Xbox One can only store up to 500 gb at the time of this post and with the external hard drive screenshot above there seems to be much more.
My guess is that this update with the external hard drive will coincide with E3. Stay tuned for more information.