Is there a way to turn off the iMac inbuilt display when using external display?

You can either turn down the brightness on the internal display until it goes blank, or you can try using a display manager like SwitchResX (http://download.cnet.com/SwitchResX/3000-2094_4-10558576.html) which can disable individual displays.

I tried that app and it just displays black on the imac display, it doesn't disable it. Does this also mean that it uses up resources sending video to the imacs "black" display?? If it does then a person playing a game is displaying it twice and making it work so much harder. This is extremely frusturating.

Just a guess on my part, but try Hot Corners and see if that works. (Put Display to Sleep)

Ciao.

no hot cornering the sleep mode only sleeps both displays. thanks for the reply

There is a PRAM setting you can apply that when used on laptops will prevent the internal display from working when an external is attached. It may work the same on an iMac, but there's no guarantee. To do this, open the Terminal utility and run the following command:

sudo nvram boot-args="iog=0x0"

Provide your password when prompted, and then restart your Mac to see if this does what you want. If not then you can revert it either by resetting the PRAM on your system (reboot with the Option-Command-P-R keys held immediately after hearing the boot chimes, and release them when the system resets and sounds the boot chimes again), or by running the following command at the Terminal and then restarting again:

sudo nvram boot-args=""

thanks for the info and reply, I really appreciate it unfortunately it didn't do anything. I reverted and am just using that app for now 😐. Apple really needs to have this ability in either an update or the next OSX release. The fact that other OS's can do it on my hardware leads me to beleive that it is completely posible on their part. Thanks again 🙂