Q: On boot, iMac Apple logo blinks red after update to Mac OS Sierra

Same issue here too. Sierra, Retina 4K 21.5inch late 2015

Same problem, after several calls to Apple they were not aware of this. Tried several re-installs etc and tried usual repairs, removed virus checker and pruned Little Snitch. No significant issues as yet but red box still appears. Time Machine slow and slight problem with mail but neither show stoppers.

Apple suggested I take to Repair as issue does not show on Booting from El Cap on attached disk and first level support suspected damaged disk. S.M.A.R.T. checks show disk ok. I suspect some bug that will be fixed on next update.

Finally upped to Apple second level support after I found that starting in safe mode eliminated the red box flash, and had a long discussion and help from a very knowledgeable support person. Tried eliminating dozens of extensions, startup items etc and many restarts none of which eliminated the issue of the red box. Notably did not eliminate any Apple extensions. So suggest that problem is a bug in an Apple extension that is immobilized with a safe boot.

Thanks Mike. Apple surround turning RED IS WELL DOCUMENTED. I also have at shut down a missing PROGRESS SPINNING WHEEL. Hard to tell if the computer is shut down. Also discovered, FREQUENTLY VISITED FAVOURITES will not show up at all. CAPTURE DATA WAS PLACED ON MY COMPUTER AND SENT TO APPLE for them to correct the software problem. Since dealing with APPLE I have yet to experience this product to run without issues. Dear APPLE ENGINEERS.... Create, Test then Share. It is now up to Apple.

Here too on new iMac 21.5, 4K retina, late 2015 but NOT on mid 2012 MBP. Both updated from El Capitan to Sierra at about same time - both basic set up, no "special"apps! The start up in safe mode seems to have stopped it this time.

Apparently just a Sierra anomaly, according to several people who have asked Apple - to be fixed in an update. I have it too and it doesn't seems affect anything.

Not actually a 'problem' as far as I can tell. Apparently just a Sierra anomaly, according to several people who have asked Apple - to be fixed in an update. I have it too and it doesn't seems affect anything.

Something a little different...spent a few hours with apple supprt on the phone and the conclusion was faulty hardware on the iMac 4K. This is a recent purchase for me and they advised to return. We tried resetting the ram, repairing the disk, and then ultimately a wipe and clean install of OS Sierra. None of which resolved the issue. Disappointing because I pay the Apple price premium because they usually work really well...