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The actual question: Time machine calculates the size of my backup as 925GB, while diskUtility shows only 858GB is used, while OmniDiskSweeper and GrandPerspective apps show that I am only using ~650GB. How can these amounts be different, which is correct, and why is the time machine backup significantly (~100GB) larger?

Background: I have a 2017 MacbookPro with a 1TB SSD. Recently Time Machine backups started failing because there "isn't enough space", which is odd because there is, and if there isn't it should just delete older backups to make room. This happens periodically and the only fix seems to be to completely wipe my backup drive, and start all over with a new complete backup.

Note:

  • I have seen in other answers posted that if it is the very first backup (or the second) the disk must have enough space to record the delta between the two, which in my case, it does, but is also irrelevant because there are at least 5 backups on the disk.

  • I have also made it a habit to run sudo tmutil thinLocalSnapshots / 10000000000 4 because this has been such a consistent problem

Other things I've tried:

  • I have emptied the trash

  • I have emptied the .trashes in multiple directories (sudo rm -rf /Volumes/*/.Trashes/*) per this answer:

  • Some size readouts include duplicated files twice, including Time Machine. Some won't if they're calculating "total space - free space" – Ezekiel Elin Feb 13 at 22:05

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