iCloud Drive is re-uploading everything

For what it's worth, I'm seeing the same thing, for at least the third time now in the past few months (~550K files uploading).

This is 6 days after upgrading to macOS 11.4 but no other recent changes that would have updated that many files. The machine is online for 12-18 hours each day and has been syncing files to iCloud Drive quite happily in the meantime. Based on modification dates and the "uploading" status shown in the Finder it appears to be uploading everything - even files last modified in 2015. For bonus points, the upload looks to get stuck at various points, though that may just be it uploading an especially large file.

The iCloud Drive and desktops box is turned as on and syncing has started Use iCloud Drive to store documents on your Mac and iOS devices - Apple Support

Get file, folder and disk information on Mac - Apple Support

Keep the Mac connected with high network speed and connect with power plug , it will take time to sync those files let the bar be completely loaded .

After it update What's new in the updates for macOS Big Sur - Apple Support

Keep your Mac up to date - Apple Support

geek_person wrote:

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Do you know the posible reason? maybe is doing a backup because an update (11.3) it's ready to install?

It cannot be a backup, because iCloud Drive is not a backup - there is no iCloud backup for a Mac like for iPhones or iPads.. iCloud might be syncing the complete iCloud Drive again, if your Mac has not been connected to the internet for a long time and your iCloud Drive is not in sync with iCloud. Or because the checkmark for iCloud Drive has become disabled and enabled again. There is nothing you can do. If you interrupt the new upload, it will just take longer.

How long have you not been using this Mac on the internet, so you needed to plug it in? There has been a problem with iCloud on April 20, when some people could not sign in at all.