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I have a macbook pro with OS X 10.5 connected to an external monitor. Mirroring is currently turned on. I want to set my external monitor as my primary and make my laptop LCD my secondary. How do I achieve that?

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up vote 11 down vote accepted
  1. Go to System Preferences in the Apple menu.
  2. Click Displays.
  3. Turn off mirroring.
  4. Click the Arrangement tab.
  5. You'll see a graphic indicator of the two screens. Drag the white menu bar indicator off the MacBook screen indicator onto the secondary display indicator.
  6. Close System Preferences.
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To change from desktop mirroring mode to screen spanning desktop mode use the Arrangement tab of the Display preference pane (e.g. search for “mirror” in System Preferences). There should be a checkbox for “Mirror Displays”. Uncheck it.

If you do not see an “Arrangement” tab and you are using an older machine, you might try a hack like Screen Spanning Doctor to enable multiple desktops on machines that are otherwise restricted to desktop mirroring. This should be unnecessary for you since you seem to seem to be using a MacBook (Pro?).

Once you have screen spanning enabled, you can drag the elements of the Arrangement display to affect a new arrangement. The rectangles for each display and the thin horizontal strip for the menu bar are all dragable. The menu bar can only be on one display. You might consider the display with the menu bar to be the “primary” one. For most application purposes, the combined display effectively creates a single, large desktop (the desktop “spans” the displays). One exception might be the initial login menu. I think it might only display itself on the display that is configured to have the menu bar.

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There'll be an asterix on your primary screen in the Display Preferences.

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