Starting with Mountain Lion, Time Machine includes direct support for multiple backup drives. We're going to use that new capability to build a basic multi-drive backup system. To understand how the backup system will work, we need to examine how Time Machine deals with multiple drives.
How Time Machine Makes Use of Multiple Backup Drives
When multiple backup drives are available, Time Machine uses a basic rotation scheme. First, it checks for any backup drives that are connected to and mounted on your Mac. It then examines each drive to determine if there is a Time Machine backup present, and if so, when the backup was last performed.
With that information, Time Machine selects the drive to use for the next backup. If there are multiple drives but no backups on any of them, then Time Machine will select the first drive that was assigned as a Time Machine backup drive.
If one or more of the drives contains a Time Machine backup, Time Machine will always pick the drive with the oldest backup.
Since Time Machine performs backups every hour, there will be a one-hour difference between each drive. The exceptions to this one-hour rule occur when you first designate new Time Machine backup drives, or when you add a new Time Machine backup drive to the mix. In either case, the first backup can take a long time, forcing Time Machine to suspend backups to other drives that are attached. While Time Machine supports multiple drives, it can only work with one at a time, using the rotation method defined above.
Working With Drives Temporarily Attached to Time Machine
If you want to add another backup drive, so you can store a backup in a safe location, you may wonder how Time Machine works with drives that aren't always present. The answer is that Time Machine sticks with the same basic rule: it updates the drive that has the oldest backup.
If you attach an external drive to your Mac that you use just for off-site backups, chances are it will contain the oldest backup. To update the off-site drive, just connect it to your Mac. When it appears on your Mac Desktop, select "Back Up Now" from the Time Machine icon in the menu bar. Time Machine will update the oldest backup, which is likely to be the one on the off-site drive.
You can confirm this in the Time Machine preference pane (click the System Preferences icon in the Dock, then click the Time Machine icon in the System section). The Time Machine preference pane should either show the backup in progress, or list the date of the last backup, which should be moments ago.
Drives that are connected and disconnected from Time Machine do not have to go through anything special to be recognized as Time Machine backup drives. Just be sure they're mounted on your Mac's Desktop before you launch a Time Machine backup. Be sure to eject the off-site drive from your Mac before turning its power off or physically unplugging it. To eject an external drive, right-click on the drive's icon on the Desktop and select "Eject (name of drive)" from the pop-up menu.
Restoring Time Machine Backups
Restoring a Time Machine backup when there are multiple backups to choose from follows a simple rule. Time Machine will always display the backup files from the drive with the most recent backup.
Of course, there may be times when you want to recover a file from a drive that doesn't contain the most recent backup. You can do this using one of two methods. The easiest is to select the drive you want to display in the Time Machine browser. To do this, option-click the Time Machine icon in the menu bar, and select Browse Other Backup Disks from the drop-down menu. Select the disk you want to browse; you can then access that disk's backup data in the Time Machine browser.
The second method requires unmounting all Time Machine backup disks, except the one you want to browse. This method is mentioned as a temporary workaround to a bug in Mountain Lion that, at least in the initial releases, prevents the Browse Other Backup Disks method from working. To unmount a disk, right-click on the disk's icon on the Desktop and select "Eject" from the pop-up menu.