When you plug in a second monitor to Macintosh computer, it may display an exact copy or ‘mirror image’ of what is on your first display. This is called mirroring. This is fine if you are doing a presentation and you want to see on your laptop screen exactly what is on the projector, but it’s not very useful at home to have the same thing on both screens. If you’d like to see different things on each screen, so the second screen gives you more desktop space, you need to turn mirroring off. Here’s how.
1.Go to’ system preferences’ in the Apple menu, and then to ‘ displays’ and click on the ‘arrangement’ tab.
2. Deselect the box that says ‘ Mirror Displays’
3. You have now turned off mirroring and should have two separate desktops!
On some machines, for example lower end of iBooks and iMacs, to differentiate them from the higher models, Apple have disabled the ability to have two separate displays so that your external monitor is permanently mirroring what is on your main monitor.
There is a program that can re-enable this feature to turn on the ability to have a separate second display. It’s called screen spanning doctor. It automatically checks that your Mac is able to run a second monitor and then makes the appropriate changes. You can download it from here.