I had a problem like that once with one of the newer Nuvi's. I had to switch a setting on a Developer Menu to MTP or USB mode to get it to behave like a mass storage storage device. Can't recall how I got to the menu but I copied this from a Garmin website.

1. Start NUVI by itself and go to Volume Screen.
2. Press the upper right hand corner of screen for 10 seconds.
3. The Developers Screen will pop up
4. Scroll down to MTP SETTINGS
5. Change from AUTO DETECT to MASS STORAGE
6. Shut down NUVI and connect to PC
7. It should now connect in mass storage mode.

As an alternative to step 1 above, you can bring up the Nuvi's "dashboard" screen, then press and hold the speedometer for about 10 seconds to access the developer screen.

I think I used the Alternative method.

Good luck