The first critical thing to have is a working FRESH install of your current OS on anotherdrive, like a spare external hard drive. No explicit instructions, but I hope the following info is useful: Apple really need to manage that site in a practical and functional manner instead of merely policing it for Apple-preferred attitudes.

it seems I’ve discovered why there are so many generic, unhelpful, and pathetic, replies on that site. I see how the site “rewards” people with higher ranks for various activities, and he has a high rank. He even seems to have marked his reply as “helpful” (so I did the same to my followup what the hell kind of help site lets the poster mark their own posts as helpful!?!?). I posted this to Discussions.Apple dot com, and today I found some doofus had added a very poorly written followup that shows he didn’t pay a bit of attention to my post. I don’t know if this will work for you, but this did work for me after everything else failed to, and it did NOT require a user’s entire system to be reinstalled from scratch! Their only other advice is a full system reinstall, which is NOT necessary, so long as you have access to a fresh (or working) OS boot volume with the appropriate freshly installed root /Library/Keychain content (mine was an external USB hard drive that I installed High Sierra on for testing and troubleshooting a previous iCloud sync issue). A reinstall over the existing OS does NOT fix this problem because it does not replace the root /Library/Keychain. They will only advise you to reinstall, or wipe out the system with a fresh installation. This hardware and software aren’t supported by Apple. NOT your user Keychain found in your user Library. Note: that’s the Library Keychain found in the root of the drive. If you encounter errors logging in to iCloud, especially after a failed, or forced, log out of iCloud on your Mac (for instance, if you followed the advise found around the web for using “defaults delete MobileMeAccounts”, and are left with a machine that can’t log in to iCloud due to getting the message in the title), the solution (in my case) may be for you to replace the /Library/Keychain content with that from a FRESH OS install. This is me trying to offer useful info to the community for any users who may find themselves in my situation.

Backup mac to icloud macos 10.12