Seems to be a USB problem, which often happens especially if on Windows PC you use the same USB-port with different devices ( Navman, camera, etc, pp).
Do you get an error message, and if, what's the exact error message you are getting, and when do you get this message?
Is the unit recognized in PC's "Devices and Printers" as a Microsoft Mass Storage Product (without yellow exclamation point, of course)?
Do you use USB 2.0 or USB 3.0?
See also here:
And also here:
EDIT:
You always could try as following in Windows 8/8.1 in hope to get rid of this:
Disconnect your Navman from PC,
remove all USB devices in Device Manager ( I think it will be best if you uninstall all USB devices),
power off PC,
wait some seconds,
power on PC,
if PC completely has booted, connect your Navman to PC, and press Navman's power button for 2 seconds to turn on your Navman,
on Navman select the Computer option from the pop-up message screen.
Windows will register the Navman and install and load the correct USB-driver.
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