Q: Red and green square dots?

You have the same machine as I - except I'm at 32Gb of RAM

Finally capitulated and baked my 5750 card today...which appears to have resolved the video/graphics issues (as no pixel have reappeared) - but my machine won't restart.

Truth-be-told, it had become progressively harder to run - due to the video problems - and got to the point where I couldn't restart it. Assuming it was due to the video card issue, I finally baked this afternoon. Now - restarting in repair mode - it initially refused to recognize the 2TB main drive, only finding the eDrive... when I tried to reinstall system software.

Finally able to identify and assign the main HD as the start-up disk, but when restarting it refuses to accept the command-R directive and enter repair mode.

Suggestions?

Possible to connect with another of my iMac's and restart in "target" mode to run utilities from that iMac?

TechTool Pro 8 has been running for over 24 hours on the OEM Seagate hard drive: 670,000 blocks (out of over 3 trillion) and more than 7,000 report "bad".

Hopefully, I'll be able to recover data and files, but it appears the drive itself is beyond salvation. Seagate 4Tb replacement en route.

Can't help but believe the HD 5750 graphics card problem exacerbated and accelerated this hard drive failure...

Good news is the graphics card issue seems to have been rectified by "the bake" (thank you - richloveOct 23, 2014 10:27 AM , from page 29 of this thread)!

I have had this problem on 2 x iMac's (27-inch, Mid 2011).

The problem on both has been determined to be the graphics card.

Apple has a replacement program, so even if you are out of warranty, you can get your mac fixed by Apple for free.

http://9to5mac.com/2013/08/16/apple-opens-graphics-card-replacement-program-for- some-mid-2011-imacs/

I wish this included the 2010 model.

As do I, Billy!

Successfully "baked" my card, but the long-term collateral stress on my hard drive caused that to fail: Replacing with 4Tb Seagate Enterprise, but not at all certain I can recover the 1Tb of data onboard....

Seems like there are many more here with mid-2010 27" iMacs, than there are with 2011's... and yet, Apple's done nothing for us!

I would just like to add another vote for baking the video card.

My story: our late-2009 iMac, around this time last year developed the problems described in this thread. What pointed me towards the graphics subsystem was the fact that when the problem manifested itself (weird graphics artifacts and in the end a frozen screen), the computer hadn't actually crashed; it was still reachable over the network, and I could in fact simply log in with ssh and reboot that way. Also, booting and quickly turning off the screen allowed it to run perfectly stable. So, I spent some time finding a replacement card, put that in, and for around 6 months all was once again well - until the problem reappeared of course.

By that time, I had found this thread, and after reading it, it seemed to me that the only solution that worked consistently was baking the card. Fortunately I had kept the old card, so I put that in the oven for 10 minutes at 205 ºC to reflow it. It even smelled nice - although that may have been the banana bread that was in the oven just before. I have just put it back in the iMac, it restarted fine right away, and I am now typing this on it. Before I removed it, this card would hardly allow the system to boot, so that's progress at least, and so far it is working perfectly.

It appears to me that the underlying problem is in the production process of these graphics cards. Apple, while being helpful enough I suppose, are not able to really solve the problem as all they do, and all they realistically can do, is find the defective part and replace it - not really a permanent solution when any replacement card is likely to develop the same issues.

Fortunately baking the card appears to solve the problem - the 27" iMacs really are very nice computers and I would hate to be left with a 27" headless machine because of a duff graphics card.

What's left now is to bake the now faulty replacement card as well, it's a Radeon HD 5750 with 1GB rather than the 4850 with 512MB the iMac originally came with, so it would be a shame not to use it. I'll post back with the result.

CZECH REPUBLIC ADDITIONAL WORLDWIDE..ALREADY do something about it !!!!!

Had a similar problem about six months ago while watching a movie. Didn't do anything about it because I thought it was part of the scene. Just happened about a week ago and my warranty is about to expire (Mid-2014 15" Macbook Pro). Tomorrow I'm supposed to hear back from the iStore, so will let you know wish me luck!

Follow-up: I have now baked the second card and that now works again too. Before, while it was able to boot into safe mode, it had constant artifacts on the screen and was getting worse over time. After, everything appears to be fine.

So, to summarize:

- original HD 4850 graphics card develops problem, deteriorating until the system was not able to boot anymore, replaced with HD 5750;

- six months in, replacement card develops problem as well;

- original card baked, all symptoms gone;

- second card baked, all symptoms gone.

I'll post back if either card goes bad again, but for now I'll consider my problem resolved. So: -1 vote for simple replacement and +2 for baking.

After 6 years of running without a single problem once (hardware that is) it's time for me I guess to get the same problems found in this topic. I've replaced the DVD drive once for a SSD and replaced the original harddrive for a larger one. So the screwing and stuff is not the biggest issue here.

I'm the owner of a late 2009 27" iMac with i7. And the problem now is 5 vertical yellow lines while in safe mode and from boot, and other screens can give me a white background and small yellow blocks all over the display.

I still want to contact apple support to ask them about the knowledge of this problem and what possible fixes are, but it seems that only baking is on of the better options. Maybe I'll try that and if it works again I'm thinking about selling it and save money to buy a new one or maybe even build a hackintosh instead! There are some positives avout it, especially when it cames to be able to repair faults! And offcourse prices

I'll be back in a few days with an update on my phonecall.

I also contacted someone who wants to do it for me but asks 100 euro's, a bit too much if you ask me. Offcourse it's reasonable for the labor, but it's nothing I can't do myself. He also said that reflowing is an option that can work for 1 day or maybe 3 years. Well, if I'm selling it I hope it will do 3 years before problems appear

Well, I called the Apple Store and they couldn't help me with the callback. They said I had to call a repaircenter and let it repair. Called them, they said I needed to consider a repair that could be at least 600 euros.. I guess not

So I just finished my backing, It's cooling down at the moment and I'll put it back in when possible. And then let's see what happens

I guess my problem was a heat issue. In safe mode I was seeing GPU temperatures around the 60C. Now with new paste and a "reflow" the temp is around the 30C in idle, and with some video now around the 40C.

Well, I'm very happy right now

I'm going to attempt the bake next weekend because with the winter the crashes have become unbearable. Thanks to all who have posted their success. I have yet to open up my mid-2010 27" iMac. I hope I don't break anything but I have disassembled and reassembled my 2006 Macbook many times (which still runs beautifully to this day with a SSD).

I'll report with my results.

I finally got around to baking the card for 7min at 390C and it made no difference. I'll admit it was really disappointing. Maybe I needed to bake it longer.

I replied to the wrong dude. I don't know how forums work apparently.