A:Answer Yep, the WD Easystore 5TB works just fine with the Steam Deck and the Steam Deck dock. You can plug it in via USB and the Steam Deck will see it as an external drive, so you can store games and other files on it easily.
Just a heads up — if the drive is formatted for Windows (like NTFS), the Steam Deck can usually read it but might have some trouble writing to it. If you want to use it smoothly for saving games, you might want to reformat it to exFAT or something that plays nice with both Windows and Linux.