Can confirm the Western Digital SN520 256GB NVMe drive is compatible with the Deck : r/SteamDeck
NEW Western Digital PC SN520 256GB M.2 2280 NVMe SSD SDAPNUW-256G | eBay
Western Digital SN520 256GB M.2 2280 NVMe SSD Drive – Electronics Outlet: Open Box New & NEW Electronics, TV's, Computers, Tablets, Printers and Networking Gear
SanDisk PC SN520 256 GB Solid State Drive - M.2 2230 Internal - PCI - SDAPTUW-256G
SDAPTUW-256G Western Digital PC SN520 Series 256GB TLC PCI Express 3.0 x2 NVMe M.2