Yosemite not able to use external display at full resolution
Upgraded to Yosemite 2011 13" Macbook pro.  Display port to VGA is what I used with Mavericks.  As soon as I updated to Yosemite my macbook would not recognize my external display.  Applecare support were stumped.  Got a Display port to HDMI adapter 3rd party, and voila back in business.  Yosemite does not like display port to VGA adapters.  I tried apple branded VGA adapter and nothing.  As soon as I switched to HDMI all resolutions appeared.