During today’s big Xbox Series X spec reveal, Microsoft confirmed that the mystery slot in the back of the upcoming console will be used to house a proprietary NVMe SSD card designed to increase the internal storage, which is currently 1TB by default. However, you’ll still be able to use USB 3.1 and external HDD for last-gen games.

As detailed in a Digital Foundry report on the Xbox Series X, the next-gen console utilizes “Velocity Architecture” that turns games storage into additional memory. While internal storage for the Xbox Series X will be set at 1TB, Microsoft is offering a custom solution.

Owners will be able to use proprietary NVMe SSD cartridges to increase the storage capacity. However, the SSD will only be necessary for Xbox Series X games specifically because of the need to optimize next-gen titles for Velocity Architecture.

Microsoft’s Larry Hryb revealed in a Tweet that owners can continue to use USB3.1 and external HDD to run Xbox One, Xbox 360, and original Xbox games directly from the external device. This should mean that players who already use external storage devices to house Xbox One or older generation games can easily migrate them to the Xbox Series X.

A potential downside of the proprietary NVMe SSD, however, is that buyers will likely only be able to purchase the SSD cartridges directly from Microsoft or official partners. Meaning you likely can’t just purchase third-party alternatives that could potentially be cheaper or more readily available.

With new games easily clearing 100GB of storage, space will become more valuable as game generations progress. But without an official price for the NVMe SSD, it remains to be seen what financial impact this will have on Series X owners.

Check out IGN’s Next-Gen Console Watch series for the latest in next-gen console news.

Matt Kim is a reporter for IGN. You can reach him on Twitter or email at mkim@ign.com