Here are instructions on how to have Time Machine switch between two backup hard drives.

My recommendations for setting up a comprehensive backup system for your Mac include swapping between two external hard drives. While one hard drive is being used to backup your Mac the other drive is kept off-site in a secure location. Using multiple drives provides protection against fire, floods and burglars, which could affect both your Mac and the backup drive sitting next to it.

Update Oct 2012 – Backing up to multiple hard drives is easier to set up if your Mac has Mac OS Mountain Lion (OS X 10.8.x) installed.  One no longer has to go through the rigamarole that I describe below when you swap between two or more hard drives. Instead, Time Machine will remember that you want it to be ready to backup to two or more hard drives. When you swap the hard drives it will automatically start to backup to the hard drive that you just attached. It’s easy to configure Time Machine to backup to multiple drives. Read this article from MacObserver for instructions.

In last week’s Tech Tip article, I described how to configure Carbon Copy Cloner to perform a full bootable backup whenever the backup drive is reconnected. [NOTE March 2024:  Creating a bootable backup with Carbon Copy Cloner has become more difficult in current versions of the macOS.  See the CCC site for details.]  If you swap your two backup drives on a weekly basis then Carbon Copy Cloner will automatically update it’s full backup each week. In other words, Carbon Copy Cloner can be configured to backup to multiple drives. On the other hand, Time Machine can not be configured to backup to multiple drives. Instead, each time you switch your hard drives you’ll need to manually select the currently attached drive and configure Time Machine to use it.  The good news is that Time Machine will look at the attached hard drive and see that it has stored backup data on this drive previously and will pick up where it left off. It’s important to understand how Time Machine performs its backups.

Time Machine performs automatic incremental updates. Time Machine is scheduled to run automatically every hour as long your Mac is turned on and awake. The first time it performs a backup it can take many hours since it is backing up all files on your Mac. It actually skips some files including items in the Trash, cache files, log files and a few other files. Subsequent backups take much less time since Time Machine only needs to backup modified and new files. It’s also useful to know that Time Machine doesn’t hold onto every hourly backup forever. It performs some automatic deleting of backups. It saves hourly backups for the last 24 hours. Beyond that it saves daily backups for the past month and weekly backups until your hard drive is full. Once a backup drive becomes full then it deletes the oldest weekly backups to make room for new backups and notifies you when it does this.

When you swap backup drives, Time Machine performs a scan to determine what files are new or have been modified since the last time Time Machine performed a backup to a particular drive. In our case, when you swap the drives, the most recent backup would be about a week ago. Time Machine thus performs an incremental backup of all files that are new or have been modified during the past week.

Specifically, here are instructions on how to re-configure Time Machine each time you swap your backup drives. Let’s assume you have two backup hard drives named “Time Machine Red” and “Time Machine Green”. For simplicity, I’ll just call them Red and Green.

1. Let’s assume that Green is currently connected to your Mac. In this case, you’ll see an icon named “Time Machine Green” on your Desktop.
2. Drag this icon to the Trash this tells the Mac to unmount (logically disconnect) the hard drive.
3. It’s now safe to physically disconnect the hard drive from your Mac. Unplug the FireWire or USB cable and power cord, if present, from Green.
4. Connect the power cord, as needed, and FireWire or USB cable to Red.
5. In about 15 seconds you’ll see an icon appear on the Desktop named Time Machine Red. (If your drive is partitioned for use with Carbon Copy Cloner then you’ll see two icons, one for each partition.)
6. Go to the Apple Menu, select System Preferences and click on the Time Machine icon.
7. Click Select Disk
8. Select Time Machine Red from the list of available hard drives.
9. Click the “Use for Backup” button
10. Close the Time Machine Preferences window. Time Machine will start to perform its backup shortly.

Whenever you swap between Red and Green simply follow these same instructions but replace every occurrence of Red with Green and vice versa. Once you’ve done the swaps a couple of times you’ll see that there’s nothing to it.