correct use of external monitor with laptop

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http://forums.fedoraforum.org – hi. i've just installed fedora 18 (default options) on an acer aspire 5735 laptop. i use an external monitor with this laptop so the sequence of events when turning it on are as follows : open laptop press power button close laptop as the laptop boots i see the grub menu and the luks password prompt on the external monitor. when it gets to gdm the external monitor screen goes grey which i've subsequently found to be because the internal laptop screen has been enabled and is displaying the greeter. once i've get past the greeter i can see the desktop on the external monitor by disabling the (HowTos)