If you're looking to expand on the Xbox One's 500GB internal hard drive, you'll have to wait at least a little while. On a PAX podcast, Microsoft's Major Nelson said that the company was falling behind on its plans to support external drives. "The future plan is definitely to support external storage, much like we do on the Xbox 360," he said, responding to an audience question. "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."
It's hardly a surprise to see some features get delayed at a console launch, though we don't know why it wasn't possible to implement it earlier, and there's no sign that it won't be coming soon after, since we're still months away from the launch. Storage woes also tend to come later in a console's lifespan, both because it takes time to build up a game library and because game sizes can increase dramatically over time. This does mean, however, that early adopters will have to stick with the standard 500GB option until the feature is added — Microsoft previously touted external hard drive support because its internal drive was non-replaceable.
Update: A Microsoft spokesperson confirmed to The Verge that external hard drives won't be supported on day one. "While not available at launch, we are working to bring the ability to add storage using a USB 3 attached external drive to Xbox One. However, on launch day, every Xbox One will include a built in 500GB hard drive and unlimited storage in the cloud," wrote a representative for the company.