My 550 locked up one time and required my pulling the battery. The reset wouldn't work nor would hooking it to power or a PC. Some faults require you pull the battery to restart it.

 

I was told by the Garmin Tech Support that the lock up occurs when switching from external power to Battery and occurs when you switch off the supply then hit the kill button on the Zumo. Their solution was to remove the battery, not exactly the answer expected for a product at this price point.

Other than the freeze out which I now carry an emergency Hex key for, cannot fault the 550........ Just wish Garmin would release a Lane Assist update for it.