Our first Xbox One that filled up stored 20 games and all 25 of Xbox One's apps. When we tried installing a 3GB update, it failed due to a full hard drive around the 362GB mark.
Any game or app you own will forever be available via the cloud, but the machine currently doesn't allow for streaming games, which would help alleviate some hard drive space in the future as Xbox One's library grows. Presently, external hard drives are not supported on Xbox One yet, but an update will permit it at an unspecified future date.
How full is your Xbox One?
Hopefully as more games come out in the near future Microsoft will get out an update for USB 3.0 external support for larger HardDrives.