Sonoma, iCloud Drive, and Time Machine
Sonoma fixes a long-standing bug in whether local files can be ‘evicted’ to iCloud, or the download removed from your Mac’s local storage. In some circumstances you could find Sonoma downloading everything you have removed from your local storage, which may come as a surprise.
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If your Mac has Optimise Mac Storage turned off but has files or folders in iCloud Drive bearing that distinctive icon, then it means that all those files and folders are likely to be downloaded to your Mac’s local storage as soon as it’s upgraded to Sonoma. You therefore need to consider whether this might cause your Mac to run low on free disk space, and plan accordingly.
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Until the release of macOS Sonoma 14.0, there has been a bug in the Optimize Mac Storage setting, that has allowed eviction of iCloud files from local storage when that setting is off, and many of us have come to accept that we can still use that feature, even though it shouldn’t have worked.
Some who have upgraded early to Sonoma have reported that, although their Mac’s settings hadn’t changed, with Optimize Mac Storage turned off, their Macs have downloaded fresh copies of every file stored in iCloud Drive, sometimes taking several days to complete. In some cases, while their Mac has still been trying to complete that large sync, Time Machine has been unable to complete any backups, reporting that those files were still synchronising.
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Stopping your Mac from syncing with iCloud is, at best, only going to postpone the problem, so that should also be avoided if possible. […] Turning Optimize Mac Storage on could help, by not trying to download a copy of every file in iCloud Drive.
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Once iCloud Drive has finished syncing, Time Machine should be able to complete a backup, including all locally-stored copies of files in iCloud Drive. If that still leaves some files stored in iCloud only, you can then download them manually ready for the next backup.
Update (2023-12-19): Marcin Krzyzanowski:
macOS Sonoma changes the iCloud file format, and as a result, everything is supposed to re-sync. This is a little ridicule that it takes weeks now and seems to never finish. I can upload 500MB in a few minutes to cloud storage, so that's not bandwidth.
