If in your nuvi ,SD content cant be processed normally ,because it doesn't reach the needed stage at boot, you need to enter preboot mode to cure it.
Or with kunix's cure ,or by other methods like the one dasilvarsa was referring to in his post above.

Trying to restore backup do you get something in update.log in sd?

Usually if nuvi reaches Logo at start it can go in preboot, please try again.
1. Turn off the navigator.
2. Press the upper left corner of the screen.
3. Turn on the navigator still holding the upper left corner of the screen.