The launch model of Xbox One included a 500 GB internal hard disk drive.
Microsoft released the Xbox One in November 2013 as the successor to Xbox 360. Every original launch unit included a hard disk because the console used it for installing games, storing downloadable content, saving media, and supporting system software. Unlike some earlier console designs, the drive was not intended to be user-replaceable through a simple external access panel.
The 500 GB capacity was available in the original Xbox One hardware, but later revisions changed the storage options. Microsoft introduced Xbox One models with 1 TB drives, while the Xbox One S and Xbox One X also came in larger-capacity configurations.
This fact is separate from the console’s external USB storage support. Xbox One systems could use compatible external USB drives for games and apps, but that did not change the capacity of the internal launch drive.