How can I extend the desktop onto an external monitor/projector?

I've plugged in a projector into my laptop and I'm attempting to extend the desktop onto it (so that I can run a full screen app on the projector and have the controls on my laptop).

I'm able to mirror the screens effectively (it does this by default) but I can't extend it. When I untick "Mirror screens" and press apply it asks me to log out and then back in again but it goes back to mirroring the screens.

I'm able to extend desktop on to my external monitor at home, just not this projector. Is there a manual way or other way to do this other that through Monitors setting?

My computer model is Dell Studio 1555: Pentium Dual Core T4300(2.1GHz,800MHz,1MB), 4096MB 800MHz DDR2 Dual Channel, 512 MB ATI Mobility RADEON HD 4570 using the ATI proprietary driver. My screen resolution is 1366x768 (16:9)

The projector that it wont connect properly is a Hitachi CPX3. That page specifies that it's especially designed for projectors that use 16:10 aspect ratio, but considering my external monitor at home uses 4:3 should the differences in aspect ratio matter or be causing this error?

Solution 1:

You can try hit win+P this shortcut configures the monitors switching between mirroring and extending the new output.

Solution 2:

There is option in System Settings Go to System Settings -> Displays
And Untick the Mirror Display option. And hit apply
You are done ! Enjoy your extended Desktop