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I have an external monitor (specifically, an HDTV) hooked up to my 2.53GHz 13" macbook pro. This display works fine and I use it with my mac in clamshell mode (eg. with an external keyboard/mouse and the laptop closed and the built-in mac screen turned off)

My Mac has multiple users on it. For User A I can use the mac with the external monitor in both clamshell and dual-monitor setups.

For User B, I can use the monitor in a dual-monitor setup, but whenever I switch to clamshell mode the Mac switches to an incorrect output resolution or frequency setting that my HDTV doesn't recognize, resulting in a blank screen and a message about Unsupported Resolution.

Chances are I did this to myself by misconfiguring my display settings at some point in the past, but I have no idea how to undo it. I (obviously) can't seen the display to change the settings when it's borked. I can see the display settings if I switch to Dual-monitor mode, but those settings only affect the dual monitor setup; no matter how I change the settings in dual-monitor mode, the clamshell mode setup remains borked.

How can I dig myself out of this hole?

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up vote 5 down vote accepted

Monitor configuration is stored in /Library/Preferences/com.apple.windowserver.plist. You should be able to delete that file entirely to return to default settings.

EDIT: Found some more. Also remove every file named ~/Library/Preferences/ByHost/com.apple.windowserver.(gobbledegook).plist.

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Good answer. Unfortunately, deleting that file and rebooting didn't fix the problem. The configuration for User A works fine, but the settings for User B in clamshell mode are still wrong. –  emmby Aug 6 '10 at 13:12
    
I've edited to add a few more prefs you can delete. Also be sure that System Preferences is closed when you're removing these files; it might re-create them otherwise. –  andyvn22 Aug 6 '10 at 15:09
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FWIW, (gobbledegook) is the MAC address of en0 by default. –  Chealion Aug 6 '10 at 19:35
    
You did it! The gobbledegook user pref file was the one that was screwed up. Thanks! –  emmby Aug 16 '10 at 16:29
    
It's a ByHost preference. They are used for roaming across different Macs when you have a shared network profile. Settings specific to the host you are on go in there. The code that appears there is a UUID of some kind for the machine in question. If you've migrated Macs, you can delete the ones from your old computer without any harm. –  CyberSkull Jun 3 at 9:37
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