Recently, one modder decided to stick a bigger 2242 M.2 SSD into their Steam Deck and after many other bigger PC sites picked it up — a Valve designer has warned people not to do it.
The Steam Deck was designed to specifically fit the much smaller 2230 M.2 and while it appears the 2242 can work, this appears to be a case of being so preoccupied with whether they could, they didn't stop to think if they should. To completely pinch a quote from Jurassic Park.
Designer Lawrence Yang said on Twitter in reply:
The modder replied to agree too.
Best to stick to the smaller 2230 series if you really wish to open it up and put a bigger one in, or just put in a good Micro SD that works well and still loads everything quickly. Don't go following every crazy idea you see online, even when big sites report on it.