MERFANI,
You are seeing the same error that I am for MyTrails. The author attempted a fix but I still have the problem that you do and the author has no more suggestions. All the other maps from this site install fine on the card unless I add MyTrails to the set. There seems to be a problem with installing MyTrails on a Mac with MyTrails itself or with the Installer 4.0. It appears to affect both the Montana and Nuvi 855.
You are seeing the same error that I am for MyTrails. The author attempted a fix but I still have the problem that you do and the author has no more suggestions. All the other maps from this site install fine on the card unless I add MyTrails to the set. There seems to be a problem with installing MyTrails on a Mac with MyTrails itself or with the Installer 4.0. It appears to affect both the Montana and Nuvi 855.