Why have my Time Machine backups stopped or become irregular?

There are several ways in which you might become suspicious that Time Machine’s backups have become irregular, or have stopped altogether. One quick check is to open the Time Machine item at the right end of the menubar, or its pane in System Preferences, to see when the last backup was run. If that was over an hour ago, give or take five minutes, then you should suspect that something is amiss.

To investigate further, you’ll need a tool which can browse existing log entries: in El Capitan, you can do that using the bundled Console app, in /Applications/Utilities; in Sierra, you’ll need Consolation or to take to Terminal’s command line using the log show command.

Your next step is to confirm when backups have been run. In Consolation, once you have started the app, just change the popup menu to the right of Period, half way down the window, to hour rather than sec, and click on the Run command button. This should then display all the log entries concerning Time Machine over the last couple of hours.

If those are empty, you’ll know that backups have stopped working. Change the number 2 next to the word Period to read 8 or 12, and press the Tab or Return key, then click on the Run command button. This will take slightly longer to complete, but should then list all the backups over the last 8 or 12 hours.

Each set of log entries should start with one reporting Starting automatic backup, and end with one saying Backup completed successfully. You may see some minor errors, such as
UserEventAgent[67]: (TimeMachine) [com.apple.TimeMachine.TMLogError] Failed to send message because the port couldn't be created.
which you can safely ignore. More significant errors, particularly any in which backup is not successful, indicate a problem occurring in Time Machine itself. You will then need to work out what is going wrong in the backup process, which could be anything from glitches connecting to your NAS to damaged file systems.

If your backups are not even starting, this is most probably a scheduling problem on your Mac. The most obvious thing to check in the Time Machine pane is that you – or a rogue app – have not turned Time Machine off. If you have, simply turn it back on, let your Mac run for a couple of hours, and check again that it is backing up hourly.

Another quick check worth making is to ensure that you haven’t rescheduled backups using Time Machine Editor, Time Machine Rescheduler, or another tool to alter Time Machine’s backup behaviour. These normally work by turning Time Machine off, then controlling backups from their own property list file installed in /Library/LaunchDaemons or ~/Library/LaunchAgents, or similar. Removing that property list file and turning Time Machine back on should solve the problem, but you should also check the utility’s documentation.

If you’re still drawing a blank and cannot see why your backups have stopped, it is time to do some deeper investigation of your logs.

tmdiag1

To check whether the backupd process is trying to run backups, switch Consolation’s top line Filter to the pattern radio button instead of Time Machine. Then in the row of popups just below that, change the first from none to processImagePath, the next popup along change to CONTAINS[c], then type backup in the text box. Leave the other popups and boxes in the Filter section as they are, change the Period to 1 or 2 hours, remembering to press Tab or Return next, and click on the Run command button.

This should fill the scrolling text box at the foot of Consolation’s window with log entries of backupd and other components of Time Machine. If it doesn’t, try increasing the Period setting until it does.

Within those log entries you may well see errors which give valuable clues as to what has prevented backup from running. You should now be able to act on those and solve your problem.

tmdiag2

If you don’t see any backupd processes, the next event to look for is initiation of backups by CTS (Centralised Task Scheduling). To do that, change the settings in the top line of popups in the Filter section of Consolation. Set the first popup (pattern) to eventMessage, leave the next at CONTAINS[c], and replace the text in the text box with com.apple.backupd-auto.

Again, the log excerpts returned in the bottom panel should give you valuable clues as to what is going wrong. Chances are that your best solution to these is going to be to restart your Mac, but the information here may give additional guidance, perhaps to restart in Recovery mode and check your startup disk, or even to re-install macOS.

tmdiag3

If you’re still not seeing any log entries, this indicates that something has gone wrong in DAS (the Duet Activity Scheduler). You may be able to retrieve some diagnostic information by altering the Filter settings again in Consolation. Make these changes in the top line of popups in the Filter section again, setting the first popup menu to read subsystem, the next left at CONTAINS[c], and the text box edited to read com.apple.duetactivityscheduler. You’ll want to reduce the period to 1 hour, no more, and then click on Run command.

This now traces the launching of a backup right back to its origin, in DAS, and should give useful clues along the way. In most cases, a restart will be needed to get DAS and CTS running properly again, and backups scheduled properly at last.

Further details about how the scheduling system works for backups are in this article.