Where is Steam writing to and where is the drive mounted to? If the drive is mounted at /mnt/games, create the folder: /mnt/game/steam, run “chmod /mnt/game/steam” and then have Steam create it’s library within there. So in the end, Steam’s folder will probably be at /mnt/games/steam/SteamLibrary. This way it shouldn’t matter what Steam is setting the permissions to.