The mass storage in valves handheld console steam deck can not only be expanded by using a micro sd card. The company relies on an m.2 slot with pci express connection. In the two more expensive versions, this is occupied by a 512 or 256 gb of gross pcie ssd, which can be exchanged. The hook: it matches 30 mm short ssds (m.2230), which are hardly reliable in retail and relatively expensive.
In the most favorable variant for 420 euros this m is.2 slot also available, ex works but blank. Valve released 64 gb emmc memory. Who buys this version, can optionally be able to reduce a faster pcie ssd.
When installing or replacing the ssd, you have to use yourself to install an operating system. For inexperienced users, valve usually offers an express installation, but so far only for the debian offshoot. Steamos 3.0, which will land on the steam deck, changes to the linux distribution arch.
Valve council meanwhile from the conversion of the steam deck and sets the micro sd slot to expand. The product page concealed the m.2 slot first even. Only after interested valve-boss habe newell had written under the official mail address, the company has proven the presence. The product page has been updated since.
180 euro for 1 tbyte
Whoever decides for an ssd change, gets two m at german shops.2230 series. Kioxias bg4 series is the faster and gentle option with prices between around 45 and 180 euros for 128 gb to 1 tbyte. Greater models are not available because only a flash memory device fits on 30 mm short ssds. Note that only the 680 euro expensive steam deck uses an anti-reflective display.
In a (video) interview with ign newell said that other manufacturers handheld consoles with steamos 3.0 can build. Similar to the steam machine concept, but that flopped. Should a company decide for this, they could meet other design decisions, such as the ssd format.