Microsoft detailed the full Xbox Series X hardware specs on Monday, confirming the nuts and bolts of the next-gen Xbox’s CPU, GPU, RAM, and internal storage — the latter of which is a 1 TB custom NVMe solid-state drive. That drive will enable fast loading times, but it also presents an issue: 1 TB isn’t much storage these days, especially as current-generation games balloon well beyond 100 GB.

Microsoft’s answer to that is the Xbox Series X Storage Expansion Card, a proprietary hardware solution being developed with Seagate. According to Seagate, those 1 TB expansion cards will replicate “the full speed and performance of the Series X’s internal storage.” The back of the Xbox Series X has a port dedicated to these Storage Expansion Cards, but the console will also accept external USB hard drives. While Xbox Series X games can be backed up to a USB drive, users must install them to the console’s internal SSD or a Storage Expansion Card to play them. This also applies for Xbox One games that get “optimized for Xbox Series X” updates, such as Gears 5 — Microsoft says those enhanced games can’t be played off of a USB drive if users want “optimal performance.”

Neither Seagate nor Microsoft revealed specifications on the Xbox Series X Storage Expansion Card (other than to say its performance “matches internal storage exactly”). The companies also did not disclose the price for those Storage Expansion Cards. We should note that 1 TB NVMe SSDs cost around $100 or more, pointing to a potentially expensive storage expansion card.

In February, when we got a peek at the back of an Xbox Series X prototype, the port that accepts Xbox Series X Storage Expansion Cards was spotted for the first time. The port was speculated to have been compatible with the CFexpress format, a high-capacity, high-speed storage option. According to Digital Foundry’s hands-on analysis of the Xbox Series X, the Storage Expansion Card is effectively an NVMe interface device encased within a chunky heatsink to keep it cool. The cards will offer some level of portability between Xbox Series X devices, letting players copy full games to the card to move them from console to console.

The Xbox Series X is slated to launch during holiday 2020.

Update (April 7): During a new episode of Inside Xbox, Microsoft noted that there will be plug-and-play compatibility between the Xbox One and Xbox Series X for external USB hard drives. Here’s how Jason Ronald, partner director of program management for Xbox, described the process: “You can easily just take the existing external hard drive that you have, unplug it from your current console, plug it into your Xbox Series X, and all your games are instantly available to you. And you can continue to run all those games directly off that external drive.”

Microsoft’s Larry “Major Nelson” Hryb reiterated that this will work only for Xbox, Xbox 360, or Xbox One games, since Xbox Series X titles must be installed to the next-gen console’s internal SSD or the Storage Expansion Card.