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Message: <blockquote data-quote="Lifeisabeach" data-source="post: 1900916" data-attributes="member: 38864">Here's an article that explains how to set the primary display in OS X. It's an older article, but it should apply to El Capitan. Basically, you need to make sure your iMac is set as the primary.[URL unfurl="true"]https://osxdaily.com/2010/04/27/set-the-primary-display-mac/[/URL]That said, the fact that the screen on your iMac keeps flaking out suggests that this problem may not be fixable. When this happens, the iMac may be switching to that external monitor automatically as using it as the new primary display. If this is the case, you likely are out of luck. But give the tips in the article a whirl to make sure everything is set right.</blockquote>