I got the following response to this little problem. Will try it out and hopefully my 520 will work again soon.
To ensure that your device is correctly connected to your PC, please do the following:-
STEP 1 Run the Registry Cleaner Tool
• Disconnect the TomTom and remove any USB devices from your PC, If you use a USB Hub, please disconnect that also.
• Run the RemoveRegEntries tool. You can download it from :-http://tomtom.com/lib/img/cs/S4S/files/RemoveRegEntries_TTGOv1.exe
A Black window will appear briefly then close.
• Make sure that you have got Administrator rights on the PC. Otherwise you may not be allowed to connect new hardware (especially the case when using office computers)
• Reboot your PC
STEP 2 Connect the TomTom
• Make sure the device is fully charged before connecting it to the computer. Ideally a mininum of 4 hours charge.
• Reset the device using a paper clip (see http://www.tomtom.com/6965)
. When resetting, keep the reset button pushed in for at least 20 seconds, but DO NOT turn the device back on just yet.
• Ensure that your TomTom has power connected to it. There should be a green light on the TomTom unit. If your device came with a Home Charger, please make sure that is plugged in to the TomTom also.
• Connect the TomTom to a USB port on the back of your PC. We strongly suggest that you use a USB 2.0 port and not a USB 1.1 Port. (USB 2.0 ports are faster USB ports, which are standard on all modern PC's, older PC's and some laptops may still use USB 1.1 ports which are slower and can prevent some USB devices from connecting properly).
STEP 3 Switch the TomTom On.
• On your PC, open My Computer
• Now switch the TomTom On while it is connected to the PC with the green light on.
• Look at My Computer, do any disk drives appear as you switch the TomTom On? If so, which drive letter is it?
• Double click on the TomTom disk.
• What files and yellow folders do you see?, please make a note of them and reply back with what files and yellow folders you see. If possible, please provide us with a screenshot showing all the details.
If the device is NOT listed as a 'Removable drive' under 'My Computer' when you switch the TomTom On, try the following:-
STEP 4 Check Device Manager
• Disconnect the TomTom from the computer.
• It is likely that you have a Firewall on your PC, in some cases, Firewalls can prevent USB devices from being recognized by the PC. We suggest configuring your Firewall and security settings to allow all USB devices to connect to your PC. Once you have done this, we suggest trying the steps above again.
• Go to 'Start' and right-click on 'My Computer' and select 'Properties'
• Select the 'Hardware' tab
• Expand 'Disk drives'. If the TomTom device is shown there with a red cross, right-click on it and select 'Enable'
• Check under 'My Computer' to see if the device now shows up as a 'Removable drive'.
We also suggest it is a good idea to try to connect your TomTom to another PC to see if you get the same problem.