Bug kills, check for user name

* Killed the "Refresh" button bug - now, when the
    user clicks on "Refresh" and the currently selected
    USB device is not yet mounted, the user is asked
    to mount the device. All sizes update appropriately
    in this case.
* Handle the lack of a user name a bit better - graceful
    error out.
* Fixed: start application, insert USB, Refresh. Now,
    the device is found and listed.
* Fixed the "available space" number