As Microsoft has confirmed that the July update for Xbox One will bring the external HDD support, here are some details about it that you might find interesting.
It was confirmed that the only requirements for Xbox One external HDD are a minimum of 256GB memory and USB 3.0. So you can pick whatever brand of HDD that you want but take in consideration this two requirements. Also you won’t be able to add media from your PC to your Xbox One via external HDD.
This is all for now, but wait to see more information that might be coming before Xbox One June firmware.