Q: iMac freezes after upgrade to macOS Sierra

How much RAM do you have installed? You can check in  menu > About this Mac. Sierra runs, but just barely, with 4 GB, but it really needs min. 8 GB to run properly.

You can also try running Safe Boot mode.

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Restart in Safe Boot mode by holding down the Shift key at startup. Safe Boot is quite slow because the OS performs some cleanup and verification tasks, so give it time. After you're completely logged in, restart normally.

I have been having this same problem, and noticed that it seems to happen while in Safari. Googling around, I found similar issue reported in El Capitan which seemed to be related to WebGL. Perhaps turning off WebGL in Safari will help? I am going to try it and see.

Update: Turning off WebGL in Safari did NOT help. It still froze up later in the day.. I think I will have to revert back to El Capitan. For the record, I have an iMac (27-inch, Late 2009) with 2.66GHz i5, and 12GB RAM.

I'm experiencing the exact same issue with an almost identical iMac.

My system is a 27-inch, late 2009 with a 2.8 Ghz Intel Core i7 with 12 GB 1067 Mhz DDR3 memory.

The hard drive has approximately 16% free space (157.05 GB of 999.35 GB).

I've booted into rescue mode and run disk utility -- the directory structure appears to be ok.

I'm not able to isolate the problem to a specific application -- to the point in the thread regarding Safari, it is running when this happens. Although it's almost always running.

I'm completely at a loss. System uptime for this machine has routinely been in the months, but I'm now forcing a re-boot at least a couple of times a day.

Since there appear to be at least 3 of us now with this issue on Late 2009 iMac's, I suggest we all report the bug to Apple. I will report it at: https://www.apple.com/feedback/macosx.html. If there are other avenues for formal bug reporting, please chime in. (PS. The frequent random freezes make my machine completely unusable, so I have been forced to revert to El Capitan.  This means I will be unable to further debug this. But if anyone finds a reliable workaround, please be sure to post. Hopefully there will be a fix at some point from Apple.

I've submitted...will see what response comes.

How did you revert to El Capitan?

I restored my machine to the last TimeMachine backup made while running El Capitan. Took 15 hours. its a complete replacement recovery, so you will need to also manually save anything you may have worked on in Sierra.

Same issue here. iMac Late-2009 as well with 16Gb RAM. Tried many things but have not figured out a probable cause yet.

Interesting. Though I don't personally have either of those apps, so it doesn't apply to my case. Please keep us posted.

Another thing we can do to get more of Apple's attention on this is to sign up for the beta test program, and then report the bug as feedback. I downloaded the latest beta, which still has this issue. You can sign up here: https://beta.apple.com/sp/betaprogram/

Once you have installed the beta, a new "Feedback" app is installed, which you can use to get the bug in Apple's formal tracker. The feedback tool also gathers information from your machine to help them debug it, so make sure you report the bug AFTER experiencing one of these freezes. The more info Apple has, and the more people who report this problem, the more likely we will get a real solution.

I had the same issue and contacted Apple support who recommended to return back the El Capitan the system does not issue a warning if there is a problem downloading Sierra it just freezes up and you need to go back to El Capitan I had 5.83 of storage left on my hard drive.