Well this is how my D600 works so I don't know if it will be the same. When I hook it to my pc / laptop by default the usb settings conform to usb modem. In order for it to act as a mass storage device I have to set it up as one in the settings menu each and every time I want this function. Also by doing this, I can only see files that are on my memory card, not the phones internal memory. I hope this helps a little.