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Apple computers: Troubleshooting issues with video on internal or external displays
I had dual monitors at the time of upgrade, once I upgraded to Mavericks, nothing changed, I still have working dual monitors. The upgrade should have nothing to do with it unless you have some sort of card inside that doesn't support the upgrade. The only thing different on mine now is everytime I start up, everything is on one monitor and I have to drag it to the right one, other than that, its fine.