But he made several (six?) guest roles - believe he won an Emmy for the Russian Summit episode.
I read somewhere that Don quit, because he thought Andy was done.
I was always under the impression he left because Disney made him an offer to star in several movies. Andy Griffith has always alluded to the fact he had "problems" during the later seasons and I have always assumed it was alcohol. The Andy Griffith character is dramatically different in the first two seasons than in the subsequent seasons and in the last few seasons he always appeared downright morose.