Larry Hyrb stated that Xbox One external hard drive support would be released very soon



From Fudzilla:

If you are an Xbox One owner like us, the problem of running out of disk space is becoming very real. While Microsoft has promised that support of additional storage via the USB 3.0 on the Xbox One is coming, we have had no real of idea of when.

Beyond Microsoft saying that they are working on it, we have heard little more about the possibility of how far along they are with the process till now. An anonymous Xbox One developer has posted a screenshot on Reddit that seems to indicate that additional storage support is coming soon. (The developer suggests that it is already in testing and it is coming soon!)

  Xbox One external HDD support in testing?