Do you use Time Machine to backup your Mac? There’s an adage that goes if it’s worth backing up once, it’s worth backing up twice. Consequently, I backup my Mac to two different hard drives as well as backing up my data to the cloud using Backblaze.

Storing backup data on two different hard drives provides me with some peace of mind since I know that these drives don’t last forever. When one of the drives starts to die on me, I don’t fret because I know I have another full backup on my second backup drive. I simply replace the failing drive with another healthy drive. Then, I’m all set.

Here are instructions for configuring Time Machine to backup to two different hard drives.

Let’s imagine that the names of these two backup hard drives are Time Machine Red and Time Machine Green.

Let’s further imagine that your Mac has been backing up to Time Machine Green for many months already.

If your Mac is using macOS 12 Monterey or older, use the instructions listed in Section 1.

If your Mac is using macOS 13 Ventura or newer, jump to Section 2.

Section 1 – if your Mac is running macOS 12 Monterey or older

1. Connect the new hard drive, named Time Machine Red, to your Mac.
2. Wait about 10 seconds to make sure that this drive’s icon appears on your Mac’s Desktop.
3. Click on the Apple Menu, select System Preferences and click on the Time Machine icon.
4. Click Select Disk.
5. Select Time Machine Red from the list below Available Disks.
6. Click the Use Disk button
7. You’ll be asked if you want to use this new disk, Time Machine Red, will replace the previous disk, Time Machine Green, or if you want to use both. Click the Use Both button.
8. Note: If this new disk is not formatted using the MacOS Extended format then you’ll be notified that this disk needs to be erased. If you see this prompt let this disk be erased, assuming you don’t have any data on this disk that you care about.
9. Note: If your second backup hard drive is in a Time Capsule or a network attached storage device, like a Synology DiskStation, then you’ll be prompted to login to the device. Enter your login credentials to connect to the Time Capsule or the Synology NAS.
10. Click the red dot to close the Time Machine Preferences window.

Section 2 – if your Mac is running macOS 13 Ventura or newer

1. Click on the Apple menu and select System Settings.
2. Click on General.
3. Click on Time Machine.
4. Click the plus sign button to add an additional drive.
5. Select your new USB external hard drive from the list of drives then click the Set Up Disk button. Important Note: Please be aware that this set up process may erase your new drive in order to change the hard drive’s formatting.
6. Click the red dot to close System Settings and Time Machine settings.

Time Machine will start to perform its backup shortly to the new hard drive, Time Machine Red. In the future, Time Machine will backup to whichever disk is currently connected to the Mac. If both disks happen to be connected then Time Machine will alternate between each drive.