About the GArminDevice.xml... Got it. I had a little error in my GarminDevice.xml:

I thought I had to use 006-B1063-01, but my hwid is 1101 so I should keep to 006-B1101-00. Also, the description is "3790" and not "3790T". It was right the first time, but had so many issues that I changed it to 3790T. The rest can stay as before I think, but adding some missing 3D map will probably help.

In the end, I just kept the .xml file and deleted everything else. Nothing was happening, si I changed all Major version to 0. He! He! myDashboard resent the GCD. Then, change back the GCD version to 2.50 and re-update again. All voice files, keyboard files and so on re-appeared. Now, I am back with the map update (which will take forever again).

Note: I am also doing all of this on my flash card and not the nuvi. I'll post a newbie guide if this ends well.