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When TimeMachine runs a backup it goes through several phases. There is the "prepare for backup", "backup" and "clean up" phases. There might be more.

What happens during the "clean up" phase?

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When Time Machine goes through the "clean up phase" it's doing "house cleaning" which will consist of activities like:

  • delete old/expired backups (i.e. past X number of days)
  • removes old(er) backups if there is insufficient space
  • trimming incomplete backups

This process may take a while, especially with many small files, as it must delete every file and folder while ensuring the backups stay consistent. For each backup to delete, it must delete all changed or removed files (compared to the kept backups) as well as remove references to identical files.

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  • How long can it be? Commented Nov 1, 2019 at 19:51
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    @NicolasMassart - It depends on a number of factors like what type of disk you're using (SSD or spinning), the interface (SATA, PCIe, USB, etc) how much stuff there is to clean up and how many incomplete backups there are. I am using a regular spinning disk connected to USB but since I do back ups religiously, it takes only a few minutes for me. Depending on how frequently you backup, your mileage may vary.
    – Allan
    Commented Nov 1, 2019 at 22:09
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    I have a spinner that for some reason always takes much longer for cleanup than for prepare and backup combined.
    – WGroleau
    Commented Sep 21, 2021 at 20:46
  • By experimenting and extrapolating, I estimate that copying everything (an initial backup) should take a bit more than twice as long as deleting an entire backup.
    – WGroleau
    Commented Sep 22, 2021 at 0:22

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