Time Machine is one of the best backup utilities available for your Mac. It should be the first thing you set up when you get a first Mac. One of the more advance features is the ability to use more than one disk. This is very useful if you have a small backup disk, or want to split backups between multiple disk in case of disk failures. It is very easy to do. Its one of the latest features found within Mountain Lion. It only takes a minute to set up.
Having Multiple Backup Disks
This setup assumes you have already formatted the disks to work with Time Machine. Open up the Time Machine preferences found within System Preferences. Hit the big button that says ‘Select Disk’. From the drop down menu select the secondary disk you wish to use.
Secondary disks can be any USB or network disk, plus any Time Capsule.
When you select the disk you will be given an option to replace the disk or use both. Select use both to join it to your current backup disk.
It is quite simple to use multiple back up disks. If you have a lot of data or a dodgy disk this may be a way to mitigate the problem.
Time Machine will rotate between the backup drives, applying the latest backup to the disk with the oldest backup on it. This ensures one of the disks will have relatively new backup.