Dudes, I created a support account just to thank RichLove for his tips. I finished a GPU bake last night and I have my iMac back! I'm running a late-2009 27" i7 iMac. Thanks for the good stuff! For everyone on the fence, just go to page 29 of this thread and follow the instructions from RichLove. Oh, and as mentioned elsewhere in this thread, the following link is great for taking the computer apart and putting it back together correctly: https://www.ifixit.com/Guide/iMac+Intel+27-Inch+EMC+2309+or+2374+Graphics+Card+R eplacement/9553

Hi Guys, before baking or doing anything please

try this fix...hope this helps!

2010 iMac with 10.10 Yosemite and Pixels

For months I’ve had a problem with my mid 2010 iMac after I upgraded to Yosemite.  I would see pink and green/multicolored squares on my screen. 

I thought “oh no, here’s a dreaded GPU issue after 4 years on my Mac hardware”.  (I wasn’t that surprised, since after five years I had issues with my first gen Macbook Pro’s GPU.)  For a while I dealt with my iMac pixel issue, puzzled why it didn’t happen all the time.

Reducing transparency did the trick.  I’m guessing there is an issue with older model iMacs and the newer transparency feature of Yosemite.  If you run into this issue after upgrading to 10.10 Yosemite, do the following:

1. Go to System Preferences, the Accessibility:

2. Click on Accessibility

3. Select "Reduce Transparency"

4. Restart Computer

I now have my iMac back to awesome working order and couldn’t be happier!  Hope this helps you..Cheers!

I do not want to play bad omen, but this trick was already mentioned on forums, so I tested it, it seems to give results for a while, but the problem does not disappear.

I wish that it works for you :-)

Even if the problem appears after updating, there is no necessarily a relation of causality. In my case this problem appeared after the replacement of the hard drive, I sought solutions, then I reinstall the disc of origin, to note that the problem was always there :-( It's the samefFor users under Yosemite, Capitan… If it's really an AMD Radeon card flaw, and it's seems to be, there is not a lot of software adjustments to make to solve it...

Alex.

iMac 21,5 in mi-2011, AMD Radeon HD 6750M 512 MB, 2,5 GHz Intel Core i5, memory 10 GB,

I know its a late reply but I have solved my iMac 27" 2010 with Radeon 5750 video card.

I had a kernel leak or something due to an error/fault in both Sophos (anti-virus) and also Little snitch (free). I deleted sophos and it was better. I still had green and purple squares on my screen and then i downloaded the little snitch installer/uninstaller and removed it. After my next full power cycle/restart it was cleared.

I had to remember what the last few programs I had altered or installed. I had both these programs installed for over 5 months. So, it took 6 months to fail on my computer.

If you think you have some kind of junk/spam/virus/firewall programs that control a lot of behind the scenes stuff, please erase it and re-install it or use something else.

12 hours in and still no more green and purple squares! The squares ended up being permanently there and changed when i vigorously waved my mouse or adjusted the screen brightness. it would eventually crash my imacs video card and everything else would still work in the background (plex, media server - music, itunes - etc)

Please Reschool, update in a few days, to let us know if it just chance... or real ;-)

Thanks

THis this above pic is what used to happen.

Now, I've Left my Mac running constantly since then (like I used to) and had zero further pixels flashing green or purple.

IT would usually have been 1/3 covered in coloured squares By now.

I Also  upgraded to yosamite. But it was doing it since half way through Mavericks Life span.

I Don't think it was the OS. I believe the programs (one or the other) have a scripting flaw (intentional or otherwise) that leak kernel resources until the video cars is overwhelmed and freezes while everything else goes on in the background. The mouse cursor would also still move but would be unable to click on anything as was also the case with the Keyboard.

I Left my Mac running since and had zero further pixels flashing green or purple.

IT would usually have been 1/3 covered in coloured squares By now.

I Also  upgraded to yosamite. But it was doing it since half way through Mavericks Life span.

I Don't think it was the OS. I believe the programs (one or the other) have a scripting flaw (intentional or otherwise) that leak kernel resources until the video cars is overwhelmed and freezes while everything else goes on in the background. The mouse cursor would also still move but would be unable to click on.

Google my solution people. i cant remember where i posted how to fix it.

Fix.

Was delete Sophos and Little Snitch. or similar. recent programs. they were stuffing it all up. Runniungsfor 3 days now since. NO MORE SQUARES!

its a kernel leak. no further firmware avail.

this really helped me thank you

Fingers crossed this worked like a charm.  I've only been back up and running for a couple hours now, but so far there's been no angry dots.

Been having the same problem last 4 months.

Tried everything I could imagine and nothing worked until I saw your post and uninstalled Little Snitch.

iMac 27" mid 2011 - AMD Radeon HD 6770M.

Thank you very much, you just saved my mental health

Very happy for you Reschool. But I don't have Little Snitch or Sophos installed... Last night I've done a clean install of el Capitan and update at 10.11.6. Work like a charm, for a few hours. But quickly squares have return, and this night a crash. Nothing more installed than OS X.

I continue to think of a flaw with Radeon card... :-(

Alex.

Maybe and maybe not - you are aware you are posting to a thread that is over 4 years old (started in April 2012 about iPhoto  under OS X 10.7.3) about totally different hardware,  OS and Photos software - if you what to discuss current issues you need to start a new thread with the specifics of your issue  --  for help is composing a good question see  Writing an effective Apple Support Communities question

LN

Dear Larry.

It's a 4 years old post about a 5 years old iMac with display problem. The first post "suppose" a Photos problem on 10.7.3. But lot of people had problem with other OS and not for Photos only.

Why give up an exchange rich in information ? Because it's 4 years old ? Nonsense !

I change my iMac, before to bake the graphic card and/or sell it, I'll test with 10.9.6 OS X for a few weeks. I will post the information here.

Because a search point quickly this thread. An exchange grows rich by the various additions, it does not age like a computer ! No ?

Alex.