Setting up Time Machine With Multiple Drives

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

Graphic of computer with multiple folders
With the introduction of OS X Mountain Lion, Apple updated Time Machine to work more easily with multiple backup drives. Alex Slobodkin/E+/Getty Images

Introduced back when the Mac used cats as operating system names (OS X 10.5 (Leopard), Time Machine is an easy-to-use backup system that has probably prevented more Mac users from losing sleep over lost work than most other backup options combined.

With the introduction of OS X Mountain Lion, Apple updated Time Machine to work more easily with multiple backup drives. You could use Time Machine with multiple backup drives before Mountain Lion came along, but it required a good deal of user intervention to make everything work. With OS X Mountain Lion and later, Time Machine retains its ease of use while providing a more robust backup solution by allowing you to easily assign multiple drives as Time Machine backup destinations.

The Benefits of Multiple Time Machine Drives

The primary benefit comes from the simple concept that one backup is never enough. Redundant backups ensure that should something go wrong with one backup, you have a second, or third, or fourth (you get the idea) backup from which to retrieve your data.

The concept of having multiple backups isn't new; it's been around for ages. In business, it's not uncommon to have backup systems that create two local backups that are 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 any reason, 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 case of fire, flood or other disasters, the business won't lose all of its data if there's a copy safe in another location. These are actual, physical backups; the idea of off-site backups long preceded cloud storage systems.

Backup systems can get very elaborate, and we won't go into them in depth here. But 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 setup not because it's ideal or will meet everyone's needs. We chose this configuration 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 Need

  • A Mac 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.
  • 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.
  • Types of drives: That's up to you. This guide will work for local internal drives, external drives, Time Capsule drives, and network drives that are supported by Time Machine.
  • About an hour of your active time to set up the Time Machine backup system, and a good deal more time to perform the first set of backups.
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Time Machine With Multiple Drives - Basic Plan

Time Machine with multiple backup disks

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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.

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Time Machine With Multiple Drives - Adding More Backup Drives

Time Machine can use multiple disks

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In this section of our guide to using Time Machine with multiple drives, we're going to finally get down to the nitty-gritty of adding multiple drives. If you haven't read the first two pages of this guide, you may want to take a moment to catch up on why we're going to create a Time Machine backup system with multiple drives.

The process we outline here will work if you haven't 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 any errors take a look at our Time Machine troubleshooting guide,

All set? Then let's get started.

Adding Drives to Time Machine

  1. Make sure the drives you wish to use with Time Machine are mounted on your Mac's Desktop, and formatted as Mac OS 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 selecting it from the Apple menu.
  3. Select the Time Machine preference pane, located in the System area of the System Preferences window.
  4. If this is your first time using Time Machine, you may want to review our Time Machine - Backing Up Your Data Has Never Been So Easy guide. You can use the guide to set up your first Time Machine backup drive.
  5. Add the drive(s) you will be using to the Time Machine exclusion list. In the Time Machine preference pane click the Options button, then use the Plus (+) button to add the drive to the exclusion list.
  6. To add a second drive to Time Machine, in the Time Machine preference pane, click the Select Disk button.
  7. From the list of available drives, select the second drive you wish to use for backups and click Use Disk.
  8. You will be asked if you wish to replace the current backup disk with the one you just selected. Click the Use Both button. This will bring you back to the top level of the Time Machine preference pane.
  9. To add three or more disks, click the Add or Remove Backup Disk button. You may have to scroll through the list of backup drives assigned to Time Machine to see the button.
  10. Select the drive you wish to add and click 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 the menu bar. You can then close the preference pane.
  13. Click on the Time Machine icon in the menu bar and select Back Up Now from the drop-down menu.

Time Machine will start the backup process. This can take a while, so sit back and enjoy your new, more robust Time Machine backup system. Or, bring up one of your favorite games. Did we mention this will take a while?