Did you go into Systems in the Control Panel and take a look at the USB ports there?
You can try to change the drive letter too:
Click Start.
Go to Settings and click on Control Panel .
Click Administrative Tools, then click on Computer Management.
Click Storage and then Disk Management
In the Computer Management window, double-click Storage, then Disk Management.
Click on the Garmin disk drive
Right-click and select Change Drive Letter and Path.
Click Edit, select a drive letter from the list (choose a never assigned letter )
Click OK
Try also to remove the sd and restart Windows with nuvi plugged in
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