Q&A: Time Machine error 28

Q My iMac running OS X 10.8.3 reports an error whenever Time Machine tries to back up to its 500 GB external hard drive. That external disk verifies in Disk Utility, with the oldest backup in July 2012, but none completed for the last six days. Backups fail in the last 0.02 GB with a log entry of ‘error 28’. How can I get Time Machine working again?

A Error 28 means that the disk or its file system has run out of free blocks. That occurs when it is full, or its internal data stores have become confuddled.

The most obvious thing to check is whether the disk is almost full, although that should generate a clearer error report. Repair the disk using Disk Utility, shut your iMac down, and power the hard disk down. After a minute or so, power the disk up, then start your iMac up.

If the error persists, reset Time Machine. Open the Time Machine pane and turn it off, once any running backup has completed. Click the Options button and record all exclusions carefully, to restore them later. Eject the external hard disk, power it down, leave for 30 seconds, and power it back up. Locate /Library/Preferences/com.apple.TimeMachine.plist on your startup volume, and move it to a different folder such as Documents in your Home Folder.

Then open the Time Machine pane again and set it up as it was before, to back up to your external drive, with the same exclusions and options that you recorded. After you have turned Time Machine back on, there will be a 120 second countdown before a new backup is attempted. This should be accompanied by re-loading of the details of existing backups on your external drive; this does not force a new backup series to be started, nor are your existing backups trashed.

If the ‘refreshed’ backup returns the same error in the same way, it suggests that there is a problem with the formatting of your external drive. The best thing to do next is to turn Time Machine off again, and make a complete copy of your existing backups on another external drive, for instance using Carbon Copy Cloner, SuperDuper! or similar.

Initialise the 500 GB drive using Disk Utility, clone the backups back to it, then reset Time Machine again and connect it back to your external drive.

Comments Time Machine is accessible via the command shell tool tmutil. Use that to inherit a backup from another Mac with the command
sudo tmutil inheritbackup
giving the path to the Backups.backupdb folder on that drive, or to associate a volume using
sudo tmutil associatedisk -a /
followed by a space and the path to its latest backup.

Further details on Time Machine are in this article.

Updated from the original, which was first published in MacUser volume 29 issue 08, 2013.