Mac OS X Lion introduces a new TimeMachine backup feature for the MacBook Air and Pro notebooks, called MobileBackups. It is a local version of TimeMachine backups that enables local snapshots of your data while you’re mobile and away from your main TimeMachine backups. On the TimeMachine timeline screen, local backups are shown as white ticks, while main backups are purple.

This is a neat feature, however it eats up your hard disk space and can grow to tens of gigabytes very fast. Thankfully, you can disable it permanently or temporarily by using the tmutil program.

To disable the local backups, open your Terminal and enter this command:

sudo tmutil disablelocal

To enable the local backups, enter this command in your Terminal window:

sudo tmutil enablelocal

Running these two commands in succession will reset your local backups by deleting them first, and then starting from the scratch. You can also use the latter command to enable the local backups on desktop Apple computers that do not have this feature enabled by default.

Behind the stages, local backups are run by the mtmd and mtmfs daemons, that mount the /Volumes/MobileBackups, a mtmfs filesystem, over a local NFS share of /.MobileBackups on your system disk.