Hello Jim-airbook,
Thank you for using Apple Support Communities. It's my understanding your iMac won't successfully shutdown after updating to Sierra. I want to make sure you're able to completely shutdown as expected.
First, I recommend resetting the System Management Controller (SMC). Follow the steps below to reset the SMC on your iMac.
- Shut down the Mac.
- Unplug the power cord.
- Wait 15 seconds.
- Plug the power cord back in.
- Wait 5 seconds, then press the power button to turn on the Mac.
Reset the System Management Controller (SMC) on your Mac
If the issue persists, test the issue in safe mode. Starting in safe mode allows your Mac to run certain checks, prevent startup items from automatically launching, and delete cache. Follow the steps below to start in safe mode.
- Start or restart your Mac.
- Immediately after you hear the startup sound, press and hold the Shift key.
- Release the Shift key when you see the Apple logo appear on the screen.
After the Apple logo appears, it might take longer than usual to reach the login screen or your desktop. This is because your Mac performs a directory check of your startup disk as part of safe mode.
To leave safe mode, restart your computer without pressing any keys during startup.
Try safe mode if your Mac doesn't finish starting up
Cheers!