You may have a better response re-posting your question in the DNS-325 board, which is more heavily trafficked than the DNS-345 board.
I personally haven't tried using a USB device in the manner you are looking for. As a test, I mounted a thumb drive in the rear USB port and the DNS-345 recognizes the device, but I'm not sure how (or if it's even possible) to publish the volume to make it visible as a network map. In the web UI under Management > USB Devices > USB Storage Information I can see information about the USB drive, but do not know where/how to directly access the content other than using the USB Backup option to copy the contents locally to the DNS-345.