With the launch of such additional features as Xbox Live and The PlayStation Network, which allow gamers to download games, apps, DLC, and other features directly to their system hard drive, the prevalence of external storage in gaming has increased exponentially.
However, Microsoft has confirmed that, at least at the time of the Xbox One launch, gamers will not be allowed to add external storage to their Xbox One systems. According to a Major Nelson podcast, when fielding a fan’s question at PAX he said, “The future plan is for 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 definitely is on the list. I don’t know when it will come in though.”
The Xbox One will come with a non-replaceable 500 gigabyte internal hard drive. Hopefully this will be sufficient until such time as external hard drive storage is supported.
Source: Major Nelson