Time Machine is a useful feature in OS X that lets you back up your Mac to an external hard drive, or to a network drive. It works automatically, every hour, backing up your Mac so you don’t have to think about it.
But sometimes Time Machine can go wrong, and your backups can get corrupted. To guard against this, it’s a good idea to back up to two different disks. These disks can both be connected to your Mac, or can both be network volumes, or you can use one of each, as I do.
To add another disk to Time Machine, open the Time Machine pane of System Preferences. Click Select Disk, and choose the second disk you want to use. When you do this, you’ll see a dialog asking if you want to replace the existing disk or use both. Click Use Both to tell Time Machine that you want to use the second disk along with the first.
When you do this, Time Machine alternates which disk it uses each time it runs a backup. In the Time Machine preference pane, you’ll see something like this:
The top disk, with the green icon, is a local hard drive; the bottom disk is a network drive. In the above screenshot, I’ve just started a new backup to a network drive, and, as you can imagine, it’s going to take quite a long time to send all that data over the network.
Since you really do want hourly backups – at least if you’re working on important files – you might want to stop the backup every now and then. Click the x to do this. When you stop the backup to the network drive, Time Machine will pick up in an hour, sending the backup to the other drive (in this case, the local hard drive). An hour later, it will continue the backup to the network drive. You can also click the Time Machine icon in the menu bar, if it’s visible, and choose Backup Now to restart the network backup.