Unfortunately traditionally you have only been able to back up to one Time Capsule or one external hard disk at a time using Time Machine. This used to mean – that at least for some of the time – one of your Macs was not up-to-date.
Fortunately Mountain Lion now sweeps this issue aside with one click of the mouse.
Perhaps your scenario is something like this…
You have an iMac at home connected to Time Capsule or an external hard disk and you are dutifully using Time Machine to back up your data on an hourly basis.
Further suppose that you have a laptop that spends some of its time away from home at the office or on business trips and while on these forced vacations it doesn’t get backed up.
Now with the Mountain Lion installed on both of these Macs you can set it up to back up to multiple hard disks or multiple Time Capsules or a mixture of both.
1. Click on the Apple menu and choose System Preferences.
2. Click on the Time Machine icon.
3. Click on the Select Disk button and choose the disk or disks you want to use for Backup.
The Second hard disk could be a Time Capsule or an external hard disk at your office. When your laptop is away from home it will back up to this device and then when you return home it will see that the office disk is not available and backup to your home disk instead. One way or the other your laptop is always backed up.