From https://www.defaults-write.com/setup-a-size-limit-for-time-machine-backup-volumes/ Setup a size limit for Time Machine backup volumes October 22, 2012 by Mr.R Time Machine will standard use all the available disk space on an external drive or a external server volume. You can setup a limit how much disk space Time Machine may use. Use this defaults write Terminal command: defaults write /Library/Preferences/com.apple.TimeMachine MaxSize 405600 Where MaxSize 405600 is the MB equivalent of 400GB (400 * 1024) To remove the limit: defaults delete /Library/Preferences/com.apple.TimeMachine MaxSize