Since it's just a PC using pretty normal PC parts, there's been some people brewing ideas already for hardware hacking on Valve's upcoming handheld PC, the Steam Deck. Valve has been pretty straightforward that the hardware, for all its fancy case and control scheme, is just a portable PC inside—Gabe Newell even said that you can put Windows on it, and even other game storefronts if you'd like.
A Reddit user emailed Gabe Newell with a followup question, asking if the Steam Deck will have a replaceable SSD, allowing users to upgrade the machine's storage. After all, from a technical standpoint the only big difference between the three Steam Deck models available for pre-order right now is storage capacity. Newell replied that the Steam Deck SSD was connected with a 2230 M.2 slot, meaning it's not soldered to the main board or otherwise connected in some extremely permanent way.
Notably, the Steam Deck website was later updated to say that "All models use socketed 2230 m.2 modules (not intended for end-user replacement)."
That does mean that, at least theoretically, you could manually upgrade the storage in the Steam Deck—but Valve doesn't think it's a good idea for you to try it. That probably means you'll have to significantly disassemble the Deck to access that M.2 slot, likely in a way that voids your warranty on the whole thing.
That makes it seem like the theoretical upgrade will only be for hardcore hardware hackers and computer-assembly professionals. You can go check out Tom's Hardware or Tech Radar for a bit more theorizing about breaking out the screwdrivers and soldering irons to go on the upgrade path.
Valve has been remarkably forthcoming about the hardware and internals of the Steam Deck, especially compared to most game console manufacturers. They also think they've avoided the thumbstick drift problem.
The Steam Deck has been the topic of a lot of discussion around the PC Gamer campfire, with dissenting opinions flying left and right. We've asked if the Steam Deck will be a success, to varying reply. Our Jorge Jimenez said he thought the Steam Deck was "a handheld PC you might finally buy," but Rich Stanton figured it was just a Switch without the magic.