https://wccftech.com/steam-deck-utilizes-standard-m-2-2230-slot-for-its-ssd-and-appears-to-be-user-replaceable/

Steam Deck, Valve's answer to the handheld console market, was publicly announced earlier this week. The new handheld incorporates a version of Steam OS, allowing users to play everything on Steam's extensive library of titles including AAA games.

Gabe Newell, President and Co-Founder of Valve, answered future user's questions on Reddit, including questions surrounding the SSD that is under the hood -- the M.2 2230 SSD. The M.2 2230 SSD has a small form factor but is not readily available to be purchased alone.