MacOS
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> Free space on a backup disk
Time Machine has stopped backing up because it says its backup disk is full. Why won’t it delete old backups to free up space, and what can I do?
Time Machine won’t delete some older backups because doing so would lose access to the files within them. It automatically “thins” or deletes backups so that it retains hourly backups for the last 24 hours, daily backups for the last month, and weekly backups to the first backup you created in that series.
Although you may be able to delete old backups manually, you’d be wisest not to, as that would remove files from your backup history that you’d be unable to replace. If Time Machine has been backing up to an APFS volume, then those backups are stored as snapshots, which can’t be moved or copied to another volume or disk.