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I have a VGA cable that I hook up from mac to the external monitor to make dual monitors. This used to work all the time but now when I connect them together it says "Check Signal Cable" what does that mean?

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Your vga cable might have a problem is there another machine that you can test the monitor with?

that's the best way to determine whether the problem is with the monitor or cable.

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That, or hitting some magic key on the Mac to enable the external output. (F7 on my old MacBook.) – Arjan Oct 7 '11 at 17:15

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