Imac display issues following Yosemite upgrade

My problem started in Mavericks, so you can't blame Yosemite. You can try adjusting various settings, but nothing will fix the problem permanently other than replacing the card. I'm ready to throw in the towel and bring the iMac to the Apple Store. I had a crash while ogged in remotely, the first in about a month.

I would say for the sake of "science" and our sanity that you try Clockingoff's example and Boot Camp a Windows install (not a virtual machine). If the Windows install runs fine natively on your Mac's hardware, then we can eliminate your card being a problem. All you have to lose is time (which is precious of course), and you could potentially save $150.

All I can say is I've always been suspicious of OS upgrades since the days of Windows 95 and beyond, which were always so blatantly problematic compared to fresh, non-upgraded installs (I'm assuming you upgraded to Mav). I let those suspicions slide with OS X after going through multiple successful upgrades, but Yosemite has rekindled those suspicions. There were some weird quirks when I first upgraded to Yosemite (booted into a black screen the first time after installing, thought I had bricked my system, but the good ol' restart-until-it-works trick worked). Who knows, it was okay for about a month after that, and maybe that's all I'll get now, too, but GpuTest's FurMark has been running for several hours now, and it's still kicking even with me mashing on Launchpad or Mission Control. I'll report back in a month if I've encountered no problems (and certainly sooner otherwise).

Edit: for anyone else reading this, during reinstall, I chose not to restore from a Time Machine backup just to avoid any old contamination, but rather had Yosemite completely install first, then used the Migration Assistant to import my files from the TM backup. I'm still skeptical that that avoided any contamination, but it seems to be fine so far. We'll see....

It's the card. Apple knows it's the card but the problem exists outside of the range of serial numbers they have designated for free replacement cards. Some people have taken to baking the card in an oven to fix the faulty soldering which is the cause of the problem.

It can't be the OS if, as in my case, the problem started in the previous OS.

You can read about the success people have had baking their cards in this thread:

Red and green square dots?

This is a new discovery. First, look at the bizarre pattern of the boxes. Next, notice the boxes are under the Safari window. When I changed the desktop image, the boxes disappeared.

This was taken from my iPad, with VNC Viewer controlling my iMac remotely.

I can confirm my problems started directly after Yosemite upgrade and I can literally "turn on" the problem by turning on Transparency settings in Accessibility. This is very clearly a Yosemite problem and not a video card problem (at least in my case and viewing a ton of other responses).

So the question is, is anyone from Apple going to take a look at Yosemite and transparency and fix the issue?

I hate seeing black edges around my sound display, but at least my screen isn't bugging out.

FINALLY I have managed to FIX the issue. As many of the posts in this very long thread have eluded to, Yosemite and my graphics card weren't playing nice and the moment I launched Premiere of After Effects, my dual displays on my Mac Pro would go blocky and the only fix was to restart my Mac Pro and not launch Premiere or After Effects (a little hard when I do video editing at my day job).

My Mac Pro has an NVIDIA GeForce GTX 470 1280 MB graphics card.

I have eagerly been waiting for a driver update or CUDA update, and a CUDA Driver Version: 6.5.36 just got released.

I've installed the update and NO MORE GLITCHY BLOCKS!

For anyone using NVIDIA graphics cards that use CUDA...

1. Launch System Prefs and, in the bottom row, click CUDA.
2. Click Install CUDA Update
3. Restart your Mac.

I really hope this helps some of you.

Thanks. iMacs use a different graphics card and as far as I know, you can't update the graphics card driver directly. I have thrown in the towel and am going to take my iMac to an Apple store.

My refurbished (that's how I bought it last year, from a specialised dealer) 2009-10 IMAC 24 is suffering from repeat episodes of a complete sudden and unpredictable loss of display - i.e. a full black screen - although the sound remains OK.

I have upgraded to Yosemite recently, but to be honest I'm not 100% sure if this issue was there before I did so (I think it must have had Maverick on it beforehand).

The dealer concerned says the machine probably needs a new graphics card - at extra cost of course.

This problem seems to be different to those posted here.

Futher to my recent post on grpahics card problems, I eventually took it to the local apple store who diagnosed my problem which was a dark doagonla area on left of screen to the display.

The problem was the back light failing and I ended up having the display replaced at a cost of £434 ouch

@SteveR53 I have a very good feeling that a roll-back to Mavericks for your refurbed iMac is going to be the solution. Much cheaper than a new graphics card, and judging by the length of this thread, a new graphics card might not even solve the problem.

My suggestion: Back up your stuff, format your iMac and install Mavericks.

So far, since the recent OS X update, the problem has stopped. My iMac was back to crashing regularly. I was on the verge of taking the iMac in, but now I'm going to wait a week or so.

Sadly, the issues came back for me, even with the latest version of OS X. I'm too lazy at the moment to try Bootcamping Windows to really settle the hardware/software issue. We really shouldn't have to do this, but sigh.

Interestingly enough, however, if I remote into my iMac from another Mac through Screen Sharing, it is much more stable. I.e. even though locally the iMac will graphically reflect whatever I'm doing remotely in real time (I observe it from across the room), the iMac will not lock up. Whereas if I do the same actions directly on the iMac, it will eventually lock up (and now, it reboots itself after the graphics lock up).

The visual glitches also do transmit through Screen Sharing, which to me signals it's still very likely a software issue. I'd imagine once the OS sends graphics data to the card, it just draws it, so I imagine the OS is sending out glitchy information to the card. Unless OS X somehow recalls the data from the graphics card, which to me sounds odd, but it is possible I suppose.

I've had the opposite issue. It would be mostly ok remoting in with Real VNC, but using Paralells Access almost guaranteed a crash. Access changed the resolution of the iMac display, which inevitably set off the "little squares" and then a crash.

Resolved a problem with squares&dots after upgrading 10.10.1 --> 10.10.2.

Started crying and searching for a videocard repair service in my country, but 'downgraded' back to 10.9.6 and the problem solved.

Now have perfectly-working Mac undeer any videocard load.

Try the same first.

That's not a solution for everybody, but it helped me in my condition.

I have had the little square blocks on my iMac (mid 2011 27" i5 3.1 with AMD HD6970M). I printed out the Replacement Program info that matches my hardware and have a Genius Bar appointment later this morning.

I seem to have this issue pop up intermittently and most recently after a crash reboot. The crash was after importing a whole boat load of images into iPhoto. After chewing on it overnight the import was completed but iPhoto would not quit, force-quit or stop from Activity Monitor. I had to force the computer to power off and when it came back on the pixel storm was there.

The pixels went away when I moved a Finder window over them and then away again.

My computer is left on all the time with the screen set to go dark after 10 minutes. I also have a G-Raid external hard drive that is used for my pictures.

I will see what happens at the Apple Store today...