Microsoft's Xbox One has a Samsung 500GB SATA II hard drive inside, but it's not designed to be upgraded by anyone. In fact, people who do decide to tear open the Xbox One's case to change the drive will void the console's warranty. Having said that, some modders have posted up proof that they have been able to upgrade the drive in the console to get better game loading performance.
A modder named Brian Williams (we assume he's not the NBC news anchor of the same name) posted up a YouTube video on his blog today that shows the Xbox One with a 1TB hybrid drive from Seagate, along with a 500GB SSD, again from Samsung. Both of these drives cut the load times of "Call of Duty: Ghosts" by several seconds compared to the standard drive inside the console.
Putting in a new hard drive in the Xbox One is one thing, but getting the console to recognize that a drive has more storage space to work with is something else. A person who goes by the name "Juvenal1" has posted a way to let the Xbox One make use of a drive with bigger storage space on the GitHub website, via a Linux-based tool. Once again, this is for the advanced users so proceed at your own risk.
Microsoft has already announced it plans to update the Xbox One at some point to support external hard drives via its USB ports. The console currently does not allow users to view just how much free storage space is available, but Microsoft Xbox executive Albert Penello posted word recently on the NeoGAF message board that such a feature will be added when the external hard drive support is enabled.