If your Mac worked fine with the external monitor before updating the Nvidia Graphic Update, uninstall the update would be the best solution now. And it works for me.
Requirement: you must have a TIME MACHINE
- Browse the content of the TIME MACHINE and choose the back up folder having the date before you updated the system. Navigate to /System/Library/Extension.
- Drag the following files from /System/Library/Extension on the TIME MACHINE to /System/Library/Extension on your hard drive and choose replace
AppleGraphicsPowerManagement.kext
NVDANV10Hal.kext
NVDANV20Hal.kext
NVDANV30Hal.kext
NVDANV40Hal.kext
NVDANV50Hal.kext
NVDAResman.kext.
- Restart the computer.
Hope this solution can work on your system.
You should read this website for knowing how I can know what files should be replaced.
http://blog.kristankenney.com/2009/02/01/how-to-uninstall-nvidia-graphics-update -2009/