Time Machine won't complete backups on Sonoma

I discovered that it was due to my 2 x 4-hour long large video ProRes files stored in my iPhone that was holding up the syncing process. I transferred these files directly into Mac photos, and then deleted them from my iPhone. Backup now works. MBP Pro Sonoma.

After two weeks of having the same problem, I FINALLY FIXED THIS PROBLEM. Here's what did the trick:

I have a Mac with three accounts on it, each one linked to a separate Apple ID. My main account is used every day, but the others (for my wife and her mom) are rarely used. When I opened up one of these accounts after upgrading to Sonoma, there was a red dot error showing for the iCloud settings saying that "some iCloud data could not be synced". Apparently iCloud needed a password, which I entered. The red dot error was satisfied, the iCloud sync progressed. I returned to my Mac account, and presto! Time Machine had kicked off and was running great. I also checked that I had "Optimize Mac Storage" turned OFF in all three accounts, but I don't think this had any bearing on the situation because that's how it had been all the while when I was having TM problems.

FIX for "Time Machine cannot back up because some iCloud Drive files have not yet finished syncing"

There may be other causes for this error from the one experienced. However, in my case, I kept getting this error because I have other user accounts (for which I am an executor of their estate) on my computer that have Apple IDs, but are not properly logged in. As soon as I fully signed out of the Apple ID accounts for those other users, Time Machine started backing up for me.

The lesson learned is that Apple ID credentials for all user accounts on your computer must be up to date. Note just for your own.

Thanks! This worked for me. I have an extra (secondary) user setup on my Mac so that I can test things created online using my Primary user login so that I can see how what I did reacts with a user without my managing user rights. I had used that secondary user a couple of weeks ago for only a few minutes to test something, then immediately logged out of the secondary user.

After reading this post, I logged in to my Mac with that secondary user and just let it set for a little while so it could complete whatever iCloud sync process that may have been active for that user. Once its sync was complete, the Timemachine issue resolved with the primary user.

Thanks aleksey. This turned out to be the issue for me too. My husband also has an account on the iMac and he hadn't logged in and accepted the iCloud Terms & Conditions since I upgraded to Sonoma. Once he'd done this, Time Machine worked ok.

This is how it worked for me.

Disconnected iCloud, started back-up on time-machine, 5 minutes later I re-logged in into iCloud.

The back up finished few hours later correctly.

And now is completing the today's one.

Good luck!

I was able to fix my issue. I have 3 logins on my computer. I logged into one and let it sit overnight. Then I logged into the next one and let it sit overnight. Before I could login to the 3rd it started backing up. It does seem to not want to do a Time Machine back up unless all iCloud syncing is done.

Is it me or are all our devices getting buggy these days?

Have you logged into the same Apple ID on the Sonoma computer that you had used on your previous OS that was working? If so, I'd suggest you boot into safemode which will run diagnostics and fix many issues it finds. After doing this, reboot again into your normal default account and test again.

Start up your Mac in safe mode - Apple Support (English)

27" 2020 iMac. Time Machine would not complete a backup. I just left it on overnight and it then started backing up as normal

I suspected another possible issue; having extremely large video files on your iPhone. I had 2 large videos over 1 hour long on my iPhone. These large video files could also cause the traffic logjam to iCloud. Time Machine was on hold for about a week until I decided to transfer the files separately using a usb connection to my Mac. Further I transferred other large movie files doing the same process, and then deleted them from my iPhone. I think this method might have helped solve the logjam. My guess is that the upload to iCloud isn't as fast, due to the transfer speed of my internet connection.

I haven't tried logging out and back in again yet but I hate that. You never know what will get erased and what will be duplicated. I agree they need to fix these issues. If iCloud wasn't so integral to their system I would have dumped it a long time ago.

Also tried all these and no go with new M2 MacBook Air and Sonoma 14.2.1. Apple support with senior advisor has now escalated to engineering. Was told I'm an exception and that this has been fixed in Sonoma for everyone else -- has it?

Alternative offered so far until I hear back from Apple is to wipe the new machine and start over (which I'd probably do with Ventura).

Great! Thanks for sharing that.