Display works with laptop and Mac Pro but not my iMac
I have a 27" non-Thunderbolt display (bought refurbished a couple years ago). If I plug it into my 2011 MacBook Pro or 2009 Mac Pro, it works fine. I just bought a 27" iMac, and it won't display there. Oddly enough, it has power; and three different external hard drives connected to the black-screen display show on the desktop of my iMac. Also, I can use the black-screen display's speakers.
I've tried plugging it into both of the iMac's display ports. Nothing. I've plugged another display into the same ports, and that display worked. So weird. I appreciate your insight and advice!
Is this a good question?