One of the nice new features of OS X Lion is that you can access your Time Machine backups even when you are disconnected from the drive such as on the commute to work or even while on holiday. The downside is it takes up a chunk of your HD that you might not want to give up. Fortunately, you can turn this feature on or off (it’s on by default in Macbook’s, but off by default in iMac/Mac Pro).
When the feature is on, local snapshots of the last few days’ Time Machine backups are stored on a local drive called MobileBackups. If your time machine backup disc should get corrupted for any reason, these can also serve as a second line of defence. Be careful though, these backups get replaced and updated without warning, so don’t rely on them for anything more than the last couple of days saves.
You can access these from the regular Time Machine icon. Alternatively, find them by clicking on the computer name at the bottom of the left-hand column in any Finder window, and then navigating to the MobileBackups drive.
If you want this option on your iMac/Mac Pro, you’ll have to enable it through Terminal. Open the Terminal application and paste in the following command:
sudo tmutil enablelocal
followed by your admin password and a return-key stroke (don’t expect to see anything when you type the password).
If Terminal returns to the prompt you should be good to go. Check by opening Time Machine preferences, which should now display the message that “Time machine keeps local snapshots as space permits and…” followed by the Time Machine backup schedule. If it hasn’t worked, it’ll just say “Time machine keeps:” and then the schedule.
If you wish to disable local snapshots to save disk space (mine takes up about 8GB), then just type:
sudo tmutil disablelocal