Can confirm the Western Digital SN520 256GB NVMe drive is compatible with the Deck : r/SteamDeck
Amazon.com: Western Digital PC SN520 128GB PCIe NVMe M.2 SSD Internal Solid State Drive OEM Package : Electronics
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
Western Digital PC SN520 NVMe 256GB 512GB M.2 2242 PCIe3 x 2 For Lenovo Laptop | eBay
WD SN520 M.2 2242 NVME SSD 512GB 256GB 128GB NVMe Internal solid state drive For Laptop Western Digital M.2 2242 NVME SSD - AliExpress