Question : Problem: external monitor won't achieve native resolution on MacBook Pro

While playing Postal 2 on my MacBook Pro something froze to the point where a hard reset was necessary. After it booted I noticed my external monitor (Dell UltraSharp 21") went to the same resolution as the 15" screen on the MacBook.  Its native resolution is much higher (1680x1050) so the picture is grainy at best.  I plugged the monitor in to a PC and it detects at the higher res so I know the monitor works.  I have tried a factory reset on the monitor and tried letting System Preferences re-detect displays.  It treats the external as if it were identical to the built in monitor.  How can I get the Mac to re-recognize the monitor at its native resolution?

Answer : Problem: external monitor won't achieve native resolution on MacBook Pro

You  might try changing the display preferences from mirror to span and then back again or vice versa and see if this helps. Also try temporarily pulling the little white control strip to the other display and then back again. You also might want to reset the pram by rebooting holding down the command, option, p, and r keys at the same time until it boot chimes twice. Follow this by repairing disk permissions while booted from your original diskwith disk utility and also use it to check the drive for any errors. If none of the above works then download switchrezx and use it to force the issue. http://www.madrau.com/