Your USB drive will appear under one of the main tabs in the media players "Browse" tab, under My Music & Devices - along with Radio, Internet (ie TuneIn & Slacker), Favorites and Recently Played.
when you select My Music & Devices, you can then pick USB1 or USB2 and then you'll see your USB drive, and are given further sub-menus to select: Songs, Genres, Artists, Albums, and By Folder. When you first insert and select your USB drive it may take a few minutes for everything to show up (including Album art pulled over the air). From there you can browse our USB music collection according to those submenus. Unfortunately you cannot use voice commands to pick songs/artists off the USB. Sadly, like the other portions of the media player, basic features are missing for playing back your USB music - e.g. no playlists, no shuffle