I had the same issue and called the customer support @ tomtom and resolved the issue by doing the following steps.
1.Connect the docking station (with tomtom on it) to computer using the usb cable.
2.Turn ON the tomtom
3.You will see the problem.
4.Now undock the tomtom (take the tomtom from the docking station).
5.On the bottom side of tomtom you will see a hole with reset button inside for resetting the device. Using a paper clip press and hold the reset button for 5 seconds and make sure that tomtom is off now.
6.Now place the tomtom on the dock and press the turn-on button for 3-4 seconds.
7.Now you should see that your computer detecting the device properly.