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My MacBook is currently using around 180GB of its internal drive. However, when I plug in my 250GB external drive to make backups of my Mac, I incessantly get the error "your backup drive does not have enough storage to perform the backup."

My backup drive still has 15GB free. From my understanding, older backups should be deleted to make space for newer ones. Is there any reason why Time Machine does not automatically delete the older backups to make space for the newer one?

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  • It can't do it if any one file is bigger than the remaining space. – Tetsujin Aug 27 '20 at 5:33
  • Does that mean that it makes the backup, and then deletes the old backups? How does it know how much free space to keep? Why doesn't TM go down to 1GB of free space ever? Is it possible for me to increase this "free space" limitation? Perhaps I can put a few 5GB dummy files inside my drive and delete them as is necessary? – Skeleton Bow Aug 27 '20 at 10:42
  • tbh, I just think 250GB to backup 180GB doesn't leave enough room for manoeuvring. Personally I always go for at least twice the size, though often I maintain closer to 3x. That way, this never arises, no matter how big any single file. – Tetsujin Aug 27 '20 at 10:47
  • When I click on the message, it actually says The backup disk needs 38.45 GB for the backup but only 15.70 GB are available. Select a larger backup disk or make the backup smaller by excluding files. So I guess this implies that I need free space as big as the backup, not as big as the biggest file. Unless that's what you meant by "file?" But yes, good point. I just thought it wouldn't be an issue. – Skeleton Bow Aug 27 '20 at 11:10
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    I just scanned my disk and it doesn't look like I have any files larger than about 5GB anywhere. Perhaps the backup size decreased sufficiently when deselecting the Parallels VM (isn't that technically a folder bundle anyway?). I wish the real reason was documented somewhere though, but I couldn't find anything online (hence the question :P) – Skeleton Bow Aug 27 '20 at 11:24
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If you wish to delete your biggest backups (because Time Machine won't do it automatically), you can download TimeTracker, which will calculate the size of each backup, as well as show you which files are taking up space. From there, you can:

  • decide to delete all backups of a file (by using the Time Machine UI and selecting "delete all backups of 'file'")
  • Add the large file(s) to System Preferences > Time Machine > Options > Ignored files
  • Delete an entire single backup by opening Terminal, typing sudo tmutil delete and dragging, from finder, the backup you wish to delete. It should say something like this in the end: sudo tmutil delete /Volumes/Backup\ Drive/Backups.backupdb/My\ Mac/2020-08-27-150021

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