Fix Guide: Time Machine "Backup Failed" on macOS Tahoe

Symptom: Time Machine starts a backup but fails midway with a generic "An error occurred while creating the backup folder" or "The disk is full" (even when it isn't).

Why this happens

macOS Tahoe 26.x changed how APFS snapshots are indexed. If your local "purgeable" data isn't being cleared fast enough, Time Machine thinks there is no room to create the temporary snapshot needed for the backup.

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Fix 1: Manually Delete Local Snapshots

1. Open Terminal.
2. Type tmutil listlocalsnapshots / to see your current snapshots.
3. Type tmutil deletelocalsnapshots / followed by the date string of the oldest snapshot to clear space.

Fix 2: Re-Select the Backup Disk

1. Go to System Settings > General > Time Machine.
2. Click the - to remove your current drive (this doesn't delete your backups).
3. Click Add Backup Disk and re-select the same drive. This forces a re-index of the backup structure.

Recommended Tool: Backblaze

Always have a second cloud-based backup in case your physical Time Machine drive fails.

- Get Backblaze for unlimited, automatic cloud backup that works silently in the background of macOS Tahoe.