To cure your Nuvi you must flash via Updater.exe
[Only registered and activated users can see links. ]
a Kunix's cure linked above by dasivarsa made with the right firmware.
To make this cure you must know the firmware HWID (Hardware ID), to download the right firmware for building the cure.
I havent linked any firmware ,I linked an application (G7ToWin) you can use to discover your device HWID.