Hi, i´m quite new mac user. I noticed horizontal artifact on my mid 2011 (HD6770) 21.5" iMac screen on startup. I uploaded video:
The artifact shows up at 0:29 on video. Also screen goes off and on, this is also new behavior after NVRAM reset. Is this normal? iMac works nice, no artifacts on normal use. Only couple seconds on startup, everytime. Problem started after I upgraded to Macos sierra and cleared NVRAM.
I have the exact same artifact, almost pixel for pixel, showing up on my mid-2011 27" iMac with HD 6970 GPU at the same point during startup. I assume this is right around when the computer is switching control of graphics processing over to the GPU. To anyone that knows better than I do, is that assumption accurate?
Anyway, my card crapped out a couple weeks ago and was keeping my computer from fully booting in any mode. Thinking back, the artifact showed up while I was trying everyone's typical troubleshooting solutions... i.e. NVRAM reset, SMC reset, etc. At the time I assumed it was just another symptom of my card failing, but I'm wondering now if it was actually the NVRAM reset like you were saying.
Ultimately, I ended up having to pull my graphics card and send it out for repair, and my computer seems to be functioning 100% normally right now aside from the artifact at startup. I've tried erasing my SSD and reinstalling osSierra as others suggested but no luck on preventing the artifact. I'm wondering, have you had any other issues since you posted, has it turned out to be just a minor annoyance when turning on the computer, or were you ever able to find a way to fix it?
Also, to anyone, and for what it's worth, the artifact only shows up when booting normally from my SSD. If I boot in safe mode, diagnostic mode, recovery mode, or from a bootable USB, there is no artifact. I'm almost tempted to install a copy of osSierra on my HDD so I can boot from that and see if there is still an artifact. Would that be a worthwhile troubleshooting step, or not worth the effort?