Try with this:

1.) Download this and unpack to desktop:

[Only registered and activated users can see links. ]

2.) Open folder for repair and updater inside and flash in pre boot
3.) instal prepusb and format nuvi ( you have picture beside prepusb to see how to set prep usb )
4.) after format create empty folder and add name Garmin
5.) Open folder Original and updater inside and flash in pre boot
6.) Now your Pc will see nuvi and you can now update your device with webupdater