To retrieve your device HWID if you have a backup (must have) go to the Garmin folder and open the file GarminDevice.xml
You 'll will see a line with :<PartNumber> 006-B0 827-00 </ PartNumber>
827 is the HWID.
If you dont have a back up (not good), how to get it?
Use the program G7ToWin that can see it in your nuvi in Pre-Boot and show up the HWID.
g7towinwithhelp.zip (1,3 MB)
[Only registered and activated users can see links. ]
Unzip the program in a folder and run it
Go to the program settings and select USB.
[Only registered and activated users can see links. ]
(Thanks Acleite for the pic )
Put GPS in PRE BOOT mode.
Place and hold one finger on the top left corner of the GPS screen and switch it on. (Without taking your finger off the screen even after power-up)
attach the nuvi USB to your pc.
Select the menuGPS -> Get GPS ID'