The Xbox Series X will come with a 1 TB Custom NVME SSD which is basically the fastest SSD that you can get today. You’ll also be able to buy an identical Expansion Card (that Seagate have created) that plugs into the back port of the console. This will also add another 1TB to your storage, but it definitely won’t be cheap. It looks as though the console will also support External HDDs, but it’s likely that you won’t get the same quick loading with these.
Xbox has confirmed that playing your older games on the Xbox Series X will bring improved boot and load times, more stable frame rates, higher resolutions and improved image quality. You’ll also be able to take advantage of using your Xbox One accessories as well as getting your games saves and progression.