Setting up Time Machine With Multiple Drives

A more robust Time Machine backup system just by adding a second drive

Time Machine is an easy-to-use backup system for macOS that helps prevent data loss. With the introduction of OS X Mountain Lion (10.8), Apple updated Time Machine to work more easily with multiple backup drives. The update provided a more robust backup solution by allowing you to easily assign multiple drives as backup destinations.

The Benefits of Multiple Time Machine Drives

The primary benefit of having multiple Time Machine drives is that one backup often is not enough. Redundant backups ensure that, should something go wrong with one backup, you have a second or third backup to retrieve your data from. It's a failsafe measure.

It's not uncommon for professional organizations to have backup systems that create two local backups used in rotation. The first may be for even-numbered days; the second for odd-numbered days. The idea is simple: If one backup goes bad for some reason, then the second backup is only a day older. The most you would lose is a day's work. Many businesses also maintain an off-site backup. In the event of a fire, flood, or another disaster, the business won't lose all of its data. The idea of off-site backups long precedes cloud storage systems.

Time Machine's ability to work with multiple backup drives gives you a great deal of flexibility in building a custom backup solution to meet your needs.

How to Build a Robust Time Machine Backup System

This guide will take you through the process of creating a three-drive backup system. Two drives will be used to attain a basic level of backup redundancy, while the third will be used for off-site backup storage.

We've chosen this example because it will show you how to use Time Machine's new support for multiple drives, and its ability to work seamlessly with drives that are only present temporarily, such as off-site backup drives.

What You Will Need

  • A Mac: The computer should be running OS X Mountain Lion or later.
  • Three drives: Each drive must be large enough to store the data you have on your Mac, and then some. The more space available on the backup drives, the more historical Time Machine data they can hold.
  • An hour of time: This is how long it will likely take to set up the Time Machine backup system, plus the time to perform the first set of backups.

If you only want to create a two-drive backup system, you can still use this guide. Just modify the number of drives from three to two as you work through the instructions.

This guide will work for local internal drives, external drives, Time Capsule drives, and network drives that are supported by Time Machine.

Time Machine With Multiple Drives: an Overview

Time Machine with multiple backup disks

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 would like 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. Time Machine sticks to the same rule: It updates the drive that has the oldest backup.

If you attach an external drive to your Mac that you use only for off-site backups, chances are it will contain the oldest backup. To update the off-site drive, 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 action in the Time Machine preference pane (open System Preferences and select Time Machine). The Time Machine preference pane should show either 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 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, then 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 for 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.

Time Machine With Multiple Drives - Adding More Backup Drives

Time Machine can use multiple disks

The process outlined below will work if you have not set up Time Machine before, or if you already have Time Machine running with a single drive attached. There's no need to remove any existing Time Machine drives. If you run into errors, take a look at our Time Machine troubleshooting guide,

How to Add Drives to Time Machine

If this is your first time using Time Machine, you may want to review the following guide: Time Machine - Backing Up Your Data Has Never Been So Easy. It will tell you how to set up your first Time Machine backup drive.

  1. Make sure the drives you wish to use with Time Machine are mounted on your Mac's Desktop and formatted as macOS Extended (Journaled) drives. You can use Disk Utility, as outlined in our Format Your Hard Drive Using Disk Utility guide, to ensure your drive is ready for use.

  2. When your backup drives are ready, launch System Preferences by clicking its icon in the Dock, or select it from the Apple menu.

    The System Preferences command under the Apple menu
  3. Select Time Machine.

    The Time Machine option in System Preferences
  4. Select Options.

    Options in Time Machine
  5. Select the Plus ( + ) icon to add the drive to the exclusion list.

    The plus icon
  6. Return to the main menu and click Select Disk.

    The "Select Disk" button
  7. From the list of available drives, select the second drive you wish to use for backups, then select Use Disk.

    The Use Disk button
  8. You will be asked if you wish to replace the current backup disk with the one you just selected. Select Use Both. This will bring you back to the top level of the Time Machine preference pane.

    The "Use Both" button
  9. To add three or more disks, select Add or Remove Backup Disk. You may have to scroll through the list of backup drives assigned to Time Machine to see the option.

    The "Add or Remove Backup Disk" command
  10. Select the drive you wish to add, then select Use Disk.

  11. Repeat the last two steps for each additional drive you wish to add to Time Machine.

  12. Once you finish assigning drives to Time Machine, you should start the initial backup. While you're in the Time Machine preference pane, make sure there's a checkmark next to Show Time Machine in menu bar. You can then close the preference pane.

    The "Show Time Machine in menu bar" command
  13. Select the Time Machine icon in the menu bar, then select Back Up Now from the drop-down menu.

    The "Back Up Now" command
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