Xbox posted some comments regarding the USB disk on the Xbox One. For those with a USB hard disk that is being used for games on the Xbox One, it is possible to migrate the games to the new Series X console.
The Series X console uses a 1TB SSD and a second expansion 1TB SSD can be installed. The reason the expansion is being mentioned is that already some games like Halo 5: Guardians now tops 100GB installed which is larger than the Master Chief Collection. Call of Duty: Modern Warfare also tops 100GB. Most users will likely grab the 1TB expansion within 12 months of the original purchase.
The USB hard disk can be connected on the Series X but games will have to be moved to the SSD in order to play them. The Series X machines are designed for the faster speeds of the SSD. The copying of games is limited by the USB bandwidth.
With larger and larger games its likely that the current 1TB Series X may be replaced with a 2TB version in a year or two. The expansion SSD is another possibility. SSD prices are still falling so PC and console alike are ramping them up rapidly.
Speculation about a diskless Series X console are less likely as the design for backwards compatibility generally requires the ability to load a game disk. There are vast numbers of Xbox One games available and almost all of them will work.