A Fix for Time Machine Always Making Full Backups

Introduction

At some point, I upgraded the macOS version on my iMac from Sequoia to Tahoe. I think this was the trigger that starting making Time Machine always perform full backups to my Mac Mini server. (Normally, it’s supposed to perform incremental backups.) Not only was this a lot slower, but it filled up my back-up disk and caused “disk full” errors. I Google’d, I asked LLMs, tried several things — nothing fixed it. I had to disable Time Machine backups until I found a solution.

The Fix

I eventually stumbled across this answer. I’ve distilled the fix into a script:

#! /usr/bin/env bash

set -e # stop on error

ATTRIBUTES=(
  com.apple.backupd.BackupMachineAddress
  com.apple.backupd.ComputerName
  com.apple.backupd.HostUUID
  com.apple.backupd.ModelID
  com.apple.timemachine.private.structure.metadata
)

DIR=/System/Volumes/Data

for attr in "${ATTRIBUTES[@]}"
do xattr -d $attr $DIR
done

At this point, I figured I might as well try it. So I:

1. Removed the remote disk as a backup destination.
2. Deleted the old backup.
3. Ran the script locally on the iMac.
4. Re-added the remote disk as a backup destination.
5. Kicked off a new backup.

It took several hours to do the first full backup, but after that, it started doing only incremental backups again. Huzzah!

The reason for this article is to help spread this fix around the Internet to make it easier to stumble across for someone else having the same issue.

Note that neither I nor very likely the original author of the fix can say that this fix will fix all situations that might cause Time Machine always to do full backups.