Ask Max: How can I revert OS X to a previous version?
I recently updated my 2009 Mac Pro quad-core to 10.6.3 and now my second monitor is not visible. It is not even being detected in displays in System Preferences or in More Info in About this Mac. Googling the issue seems to suggest that this is a known issue with 10.6.3.
How can I go back to 10.6.2?
The easiest way to get your Mac Pro back to an older version of the OS is to do an Archive and Install of the Mac OS. Mac OS 10.6 introduced a simplified way of doing this. Now when you chose a basic install of 10.6 on a newer version of the same OS, it will simply downgrade you without asking to to chose Archive and Install.
Before you go that route, you may want to Zap your PRAM by holding down [command] + [option] + [P] and [R] keys on start up. Let the System chime three times before letting go of those keys. You may also want to shutdown the System and then hold in the Power Button until you hear a long tone sound. The computer will restart. If all that fails, Downgrade to 10.6 from your install DVD and then use the 10.6.2 combo updater.