When doing the yearly update of my 276c for the coming MC season I read this thread (and the links to other threads) and just want to point out one thing that is easy to forget.
When updating these devices it is essential that they are connected to external power when updated. Fully charged battery is not enough! Error message of Communication error can be seen but also no error message but corrupted maps.
And I fully agree with the thread starter that the 276c/278/478 still are superior when it comes to showing details in zooming out which is important in this use.