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I have two screens. I have a MacBook Pro and just installed Mountain Lion. When I close the lid, instead of completely sleeping, it simply moves what I'm doing to the secondary screen. Please explain?

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This behavior is also present with Lion. I think it's called "clamshell mode". – frnhr Jul 30 '12 at 0:09

After Lion, closing the lid puts the machine straight into clamshell mode rather than putting it to sleep, if an external display is connected and the laptop is connected to AC power. I'm not aware of any way to change this.

See Apple's page on clamshell mode behaviour, comparing 10.6.8 and Lion:

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Apparently, you can disable this by simply disconnecting the power cord. I was having the exact same issue, and I read the link provided about closed clamshell mode. It stated that a requirement was the power cable, I unplugged mine, and voila!

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Unplug AC power, than close the lid. Mac goes to sleep. Now one can plug the cable back in, it won't wake the Mac (at least not my MBP running OS 10.8.2). – DerMike Feb 21 '13 at 10:09

That's the way it works (also on Lion); closing the lid means for your macbook, that you want to use the external monitor as primary screen.

You can press CMD+ALT+EJECT to trigger the sleep; or you can press CTRL+EJECT to display the dialog Restart/Sleep/Cancel/Shutdown.

Or, you could try to assign the primary monitor to the external monitor following this guide. I didn't try that, but I suppose that being the external monitor the primary one, closing the lid may not been intended as "use the external display as primary one", since it is already; so I suppose, it should intend the message differently. Let me know, I can't try now since my macbook is running some updates :)

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On my dual-screen setup (MacBook Air, Lion) I have the external screen as the primary one. Opening and closing the lid in this configuration does nothing other than waking up or shutting down the internal screen. – Harald Hanche-Olsen Jul 30 '12 at 11:06

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