Why is my display not fully waking from sleep?
This problem is well documented in blogs (not by Apple), however over the years I have not found a solution. I will attempt to accurately describe the problem.
I never let my 2009 27" iMac (2.8 GHz Core i7) go to sleep, only the display is set to sleep after a period of time. It does not matter how long the display waits before going to sleep. It does not matter how hot (or cool) the iMac is, nor does it matter if anything is plugged into any ports or not. This issue has also existed through at least four versions of OS X (Lion, Mountain Lion, Mavericks, and now Yosemite). I have an external display hooked up via Mini DisplayPort cable (the monitor has only a DisplayPort input) and it too acts the exact same way as the built in iMac display, meaning it will not fully wake either (more accurately, it will not show any video other than the pointer).
When I return to my iMac, I touch my trackpad (or anything else that wakes the display) and the backlight will turn on and the external monitor will also wake (as seen in the first photo) and most of the time even the pointer arrow will be visible (as seen in the second and third photo, it's hard to focus on anything with little light). The pointer arrow even moves across both displays fine. But every other pixel will be black with just the backlight leaking through the LCD panel. No key command or combinations of keys will allow me to go any further. The only solution is to force my iMac off and restart by holding the back power button for ~10 seconds and a click back on.
This has been an issue for years, and I believe it has been an issue when I had 12GB, 16GB, and 32GB of RAM installed over the years. This has been an issue before and after I installed a second and third HDD and a SSD (in the optical drive bay). I have also removed the logic board to get to the SATA cable, meaning virtually all the connections have been reseated. The same behavior still exists no matter what I have done to upgrade this iMac. Again, this issue has existed across at least the last four versions of OS X.
Does anyone have any idea to why the backlight will light up, and the external display will wake and yet the rest of the computer (GPU) will not respond? Again, the iMac itself never sleeps, only the display and the hard drive sleep, the fans always run.