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Ubuntu 12.04 can't detect external monitor

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linux-howto

http://askubuntu.com – I installed ubuntu 12.04 on my old laptop with SiS671 video card and its working fine with 1280x800 screen resolution. I also connected my external monitor and its working too but I want to turn off laptop's monitor and I can't find any references to external monitor. When I try to access System Config > Monitors it only shows "laptop" monitor. typing Xrandr on terminal results: xrandr: Failed to get size of gamma for output default Screen 0: minimum 640 x 480, current 1280 x 800, maximum 1280 x 800 default connected 1280x800+0+0 0mm x 0mm 1280x800 60.0* 1024x768 75.0 (HowTos)