Black Screen on an 27" iMac
I have a 2009 late model 27" iMac with the latest Yosemite OS X installed. It started about 6 month ago that the machine would suddenly turn black while I was using it. I had sent the iMac to a local Apple Authorized repair shop, they kept it for a week and would not reproduce my problem and gave it back to me and said the machine seemed to be fine after they ran a diagnostic test. But after I got the machine back, it started acting up again and were more and more frequently. I tried everything to no avail. Everything on the machine seems intact, just couldn't work for me. I need some help to solve this problem. Thanks.