[ubuntu] Connecting 2nd monitor causes X server crash using Nvidia drivers in 12.04

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http://ubuntuforums.org – I have a secondary monitor connected to my Acer Aspire 8930 laptop. Graphics card is Nvidia GeForce 9600M GT (1GB). My setup worked fine in all previous carnations of Ubuntu; I installed the recommended Nvidia driver via "Additional drivers" and was able to connect and use the Samsung monitor through HDMI output. The upgrade to 12.04 (64 bit) has rendered this impossible. As soon as I switch on the secondary monitor, the X server (and sometimes the entire system) crashes. The problem is identical in both Unity (3D and 2D) AND Gnome (gdm). It is, I believe, something to do (Hardware)