Major bumpage...
I recently updated my Gigabeat F40 from the 3.0 version to 3.9 and had this problem as well. The problem occurred over several different computers ranging from Ubuntu Linux ( versions 8.04 and 9.04 ) to Windows XP. It does not happen every time I disconnect the USB though.
What I've found is that, if the screen is dimmed and not showing the USB symbol when you remove the USB cord, when it lights back up and tries to show the Rockbox main menu it instead displays the error
*PANIC* mount:0
If instead I hit the touch-pad to bring the screen light back up, displaying the USB image... If I remove the USB cord at this point, it goes back to the Rockbox main menu.
I've tested over and over again and this behavior persists. So it appears that if you disconnect the USB cord while the screen is dimmed is what triggers this error.