Mac Pro 5,1 I have an Asus 27" connected via Mini DisplayPort (mini displayport to full displayport cable). And a 21" ViewSonic connected via Dual Link DVI. The graphics card is ATI Radeon HD 5870, 1024 MB. The main monitor I usually use is the Asus which I usually run from Dual Link DVI but I got the mini display port cable yesterday so I want to set up dual monitors. - If both screens are connected at the same time and I power on the computer neither screen turns on. - If I just connect the display port (Asus monitor) it doesn't turn on. - If I just connect the DL-DVI (ViewSonic monitor) it turns on. - Once I'm in Mavericks, if I then connect the Mini DisplayPort to the Asus then both monitors work. - When both monitors are working in Mavericks and I restart then both will stay working. It's only when I shut down the computer or put it to sleep that they both stop working. I called Apple and tried different combinations for about an hour trying to explain the problem over the phone. They've decided to send a techy out here to replace the Logic board or the graphics card. But I don't think the logic board is the problem because I had it replaced last week for a different issue so it's brand new. Anyone got any ideas?