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I get messages that my Time Machine backup volume is full and TM has deleted older backups. But my backup target disk is an external 3 TB HD and Finder reports that over 2 TB of it are free (only some 820 MB used)!

I am backing up the internal 1 TB SSD (850 GB in use) of my Mac Book Pro Retina to that volume. I have also reformatted the backup volume and started all over with a new Time Machine backup to that fresh and empty target. But the effect reappears after 3 to 4 days.

All my trials to find similar reports on Google failed. It seems that other users have less than 25% free on their backup target when they run into this message. The only similar question I could find was this one but it was not answered.

I am currently running OS X Yosemite 10.10.1 and the backup HD is connected through USB 3.

  • What is the layout of your external? Is it partition to separate disks? – tron_jones Dec 8 '14 at 14:04
  • The external HD contains just one partition with the default file system Mac OS Extended Journaled (not case-sensitive) which occupies the full 3TB of the disk. I have also tested a case-sensitive formatting but the effects are identical. – Jpsy Dec 8 '14 at 14:41
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Is it possible the snapshots being deleted are local snapshots (see http://support.apple.com/en-us/HT202301) and not on the external? Local snapshots start getting deleted when free space your internal volume (not the backup volume) drops below 20%.

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