On Friday, February 8, Luthansa 459, an A340-600, San Francisco-Munich, was about halfway between Labrador and Greenland when it turned around and landed in Montreal — over 1100 miles away. A snowstorm was in progress at the time. I saw a photo of it on the ground in Montreal today, Saturday that was posted to a spotter’s group. The flight was later completed.
I have no idea what the record distance is for a diversion but this one seems pretty impressive. Nobody seems to know the reason for the diversion.