Late last night it was revealed that the Xbox One X would come with a custom 1TB NVME SSD as well as the ability to expand with another 1TB through the use of another propitiatory NVME SSD (which plugs into the back port of the Xbox Series X).
It was also listed in the full spec list that the Xbox Series X would support external HDDs, but Microsoft has revealed that these will be only be used for backups and playing older backwards compatible games. It’s likely that you’ll be able to store Xbox Series X games on this external HDD, but not play from any external hard drives.
This is due to the fact that NVME SSDs are way faster than the fastest external HDDs (even then there’s a massive ran of speeds within external HDDs), and one of the key selling point of both next-gen consoles are loading times as well as the ability to run multiple suspend games at once.
You can find the full spec list for the Xbox Series X right HERE.