Good news reminder: the Xbox One supports external storage. Bad news update: the Xbox One won't support that feature at launch.
While announcing the capability at E3 back in June, consoles released in November won't let you plug in an external drive, Microsoft's Larry Hryb explaining in a podcast that "the team is working on some other things". As for when the consoles will support external storage, he's got absolutely no idea.
The Xbox One will ship with the 500 GB hard drive, all of them have that. The future plan is definitely to support external storage much like we do on the Xbox 360. My understanding is that feature will not be there at launch, because the team is working on some other things, but it is definitely on the list. I don't know when it will come in though.
Hopefully it's not too much later. 500GB is fine for now, but in 12-24 months time, heavy users are going to start hitting their storage ceiling...