I have the same problem, connect n7 via usb cable and in windows explorer it shows up in the folder tree in left margin as 'nexus 7', but the right hand pane which should show you all the existing folders on the device is completely blank?
when i try to drag and drop mp3 or video files, it comes up with an error message - "cannot copy to this location, the device has stopped responding or has been disconnected"
Nexus 7 running jellybean 4.2.1 - any idea's please?