I am experiencing what appears to be a known problem in my iMac, as described here: https://support.apple.com/en-ca/HT203787 It is the 3.4GHz quad-core Intel Core i7 with the AMD Radeon 6970M Video Card. The screen wen black the other day and now it will not start past the apple before going to a white/grey screen. The start-up chime plays and I can get it into Target disk mode and access the hard disks with no problem via Firewire/Thunderbolt on the laptop(even boot the laptop off the internal startup drive). Running the latest version of Yosemite. Zapped PRAM, reset SMC, repaired permissions. The serial # also ends in the identified model number. I am located in Bermuda so running to the Apple store Genius is impossible and there is one authorised Apple service company here. Their predictable response was it is the logic board and will cost $1600 to replace plus several hundred for their time. When asked about the GPU issue the tech said that the Apple Diagnostics was not able to start on the computer to confirm it which meant it was a logic board issue. While the GPU may be faulty as well he cannot confirm with out Apple's tool. I am curious, how are they supposed to confirm it is the card if the computer will not boot due to a graphics card issue. The tech got very defensive when I asked and told me I could pay to fix it or come and get it. It would likely cost me just as much to ship it to the US or Canada to take it to an Apple store there. It is rather frustrating, and I wish there was a way to test to see if I can run the diagnostics or if this guy is trying to get out of doing a free service. Thanks for any advice.