With Lion, Apple introduced local Time Machine snapshots. This mostly-silent feature lets your Mac use free space on your main drive to create iterative backups of your files when you’re away from your external Time Machine disk.
By default, Apple disables local snapshots on desktop Macs; the assumption is that you only need them when you're using a laptop, and that your trusty desktop machine is always connected to a Time Machine drive. But what if that’s not always the case? Perhaps you disconnect your Time Machine drive to connect other USB peripherals, or maybe you share the drive with other folks in your home. In any case, there's a way to enable local Time Machine snapshots on desktop Macs that Keir Thomas (author of the excellent Mac Kung Fu) discovered. All it takes is a quick trip to Terminal.
After launching Terminal (in /Applications/Utilities), paste in the following command:
sudo tmutil enablelocal
After you press Return, Terminal will prompt you for your account’s password. (You won’t see anything while you type.)
Thomas says that your Mac will create a new snapshot within an hour, but you can hurry things along with another Terminal command:
There’s no visual indication that Time Machine is doing anything, but you can visually confirm that local snapshots are being taken by entering Time Machine (from the Time Machine menu or the Applications folder). Normal, external-drive-stored backups are displayed with purple bars in the timeline display; local backups are instead shown in gray.
If you decide you want to turn off local backups on your desktop (or laptop) and remove them, just use this command:
sudo tmutil disablelocal
The local backup data may take some time to delete, but it’s a similarly invisible process.