Microsoft only just revealed that the Xbox Series X will support "expansion cards" that allow for faster storage, but it already has an accessory maker lined up. As you might have noticed in the hardware preview, Seagate has introduced a Storage Expansion Card for the Series X that will deliver an additional 1TB of game space while maintaining the same performance as the console's built-in SSD. If you primarily intend to play games optimized for the new system, this may be vital -- 1TB is already something of a tight fit with the current generation.
The company hasn't provided pricing or a release date, but we'd expect the Storage Expansion Card to be expensive when it ships. PCIe SSDs in this class are expensive as a general rule (Samsung's 1TB 970 Evo Plus is on sale for $200 on Amazon as we write this), and making one available in an easily removable format won't help keep the price down.
As with Microsoft, Seagate is keen to stress that you can't get away with a USB drive for Series X-optimized games. Conventional external drives just aren't fast enough -- they'll be fine for unoptimized Xbox One games as well as your Xbox 360 and original Xbox collections, but you'll be running into technical limits beyond that. Like it or not, this generation will likely be defined by pricey semi-proprietary drives, not to mention eventual revised console models with more built-in storage.