To enable you to play all tracks from a connected USB hard drive on your 851N, just follow these three steps.
Create a folder called 'Music' at the top level (root) of your USB hard drive
Move all music on your drive into this new folder
Once the USB drive is connected to your player, you will have the option to play all the tracks by navigating to Music Library > [USB Drive Name] > and pressing play on the Music folder
If you don't use a 'Music' folder and instead have your artists as the top level (root) folders, then you'll only be able to press play on individual tracks and add them to the queue individually.