Source: Microsoft
Xbox Series X and Xbox Series S pack shared custom NVMe SSD storage technology, bringing up to 40 times speed increases over the Xbox One family. The Xbox Series X ships with a 1TB internal SSD drive, while the Xbox Series S features a reduced 512GB capacity out of the box. In reality, Xbox Series X has just 802GB of space usable for games, with Xbox Series S offering just 364GB.
When your games library eventually fills the internal drive, Microsoft provides two primary routes to increase your available storage on Xbox Series X and Xbox Series S. It worked on the Xbox Seagate Storage Expansion Card, mimicking the internal SSD with identical performance. It also supports USB hard drives like Xbox One consoles, although this time with limitations attributed to slower speeds.