Technical Analysis: Time Machine Restore of Secondary Volumes

| Editorial

Apple's Time Machine, introduced with Leopard, is a welcome tool. It makes the process of backing up a user's boot drive painless. Recovering deleted files is similarly trivial. However, there is one often overlooked non-feature of Time Machine, lurking, that can cause a major headache -- the recovery of secondary volumes folded into the Time Machine archive. This article explores the pitfalls and solutions.

If you are like most Mac users, you have a Mac with one internal hard disk (or maybe an SSD!) and a single external drive used for Time Machine. If this is your configuration, then this article doesn't apply to you.

Time Machine

Time Machine. Credit: Apple, Inc. 

However, if you have an additional drive attached, either internally (say, a Mac Pro) or externally, then you're faced with the prospect of whether to back that drive up as well. The way this is done is to open the Time Machine preferences and click on the "Options..." button. That's where you can formally exclude a drive/volume from being backed up. 

By default, the backup volume itself is excluded. Otherwise, it would chase its own tail in a backup loop.

Time Machine options


If you have a second drive/volume and do not exclude it, it also will be rolled into the Time Machine archive. The problem is this:

Apple doesn't specify in its Mac 101: Time Machine tutorial a recommended procedure to restore that secondary drive/volume archive to a new drive (or other location) if the associated physical drive fails.

It's easy to see that secondary volume in the archive. Just use the Finder to successively open an archive until you see the multiple volumes. For one of my Macs, it looks like the screen shot below. Vega is the main boot drive and Canopus is an additional internal drive that's not been excluded from the backup.

Time Machine volumes

Archive of multiple volumes

There is ample documentation for how to recover deleted files from the main (boot volume) or to restore an entire internal drive on a new Mac from a Time Machine archive. The question is, in that latter process, what is done with the secondary drive/volume archive?

The answer is nothing.

To verify that, I booted from the DVD master that came with my Mac Pro and restored the Time Machine Archive to another internal drive. In this process, that drive is erased and restored to the exact state of the original drive. Unfortunately, I then had two internal drives with identical names and identical contents. Apple's gift you in that case, at reboot, is a perpetual kenel panic.

  • #1 - Kernel Panic
  • #2 - Hold Power button down until shutdown
  • #3 - Restart
  • #4 - GOTO step 1

The way out of this is to hold down the OPTION key at reboot. That enters the Open Firmware before the boot sequence can begin and displays the candidate volumes to boot from. I selected the Master boot DVD, then used Disk Utilities to reformat and rename that new, experimental drive that had the identical name and contents.

But I digress.

Nothing is done with the secondary volume in that restore process because there's no place to put it. So what's a user to do to restore secondary volumes?

Restoring a Secondary Volume

One technique that I found, back from 2007, is from a French publication. It may be out of date, and certainly did not work for me in 2010. Basically, the author advises mounting a new drive with the same name as the failed volume. Then selecting the blank drive and invoking Time Machine from the Dock. Today's entry will be empty, of course, because it's a fresh drive, but in going back in time, for that named volume, one should be able to do a restore of all archived directories.

When I tried this, I got a nastygram:

T.M. nastygram 1

Nastygram #1

Pushing through this process, logged on as root, resulting in zero files being restored. (I knew that because the free space on the new drive didn't change.) So much for using Time Machine itself.

The second technique is to identify a location large enough to hold the entire contents of that T.M. archived data. Then do wholesale copies of the desired directories out of the archive into the new location. This works because each folder in the archive contains either new files or links to files that haven't changed. So you needn't worry about copying incomplete directories. Just be cautious which dated archive you chose -- stay away from the time when that original drive/volume failed.

However, one of my directories belonged to another user, an unprivileged user. So when I tried to copy that directory in the Finder, the copy was aborted -- with no offer to authenticate. I had to finish that part logged on as root.

T.M. Nastygram 2

Nastygram #2

None of this strikes me as the kind of user experience Apple is famous for. At the very least, if Apple is going to allow a secondary volume to be folded into a Time Machine archive, then there should be a reliable function included in Time Machine to restore that secondary volume -- rather than leaving the user wondering how to proceed.

A Way Out

As in the case of iTunes broken links, for the sake of simplicity, Apple has under-documented and under-featured Time Machine to deal with additional (secondary) volumes that get folded into the Time Machine Archive.

One way around this is to do what some of us on the TMO staff do. Let Time Machine do its magic on the boot volume, but use an additional tool, for example Prosoft Engineering's Data Backup 3 for Mac to back up any additional volumes that are attached. (There are other solutions as well.) That way, you'll have a specific named data set for one or more additional attached volumes that can be identified and used for a restore. Remember to go back to Time Machine and exclude all the additional volumes you may have so they don't get rolled into the Time Machine archive -- unless you want that redundancy and feel comfortable dealing with it when the time comes.

Data Backup 3

Data Backup 3. Define your own custom datasets


Apple's Time Machine allows users to include additional volumes in the main Time Machine archive. They are included by name. However, there is no formal function within Time Machine that walks the user through the restoration of those additional volumes. The user is left to drag the desired directories out of the archive to conduct a restore, and there's no guarantee it will be 100% successful, even as an admin user.

When users have more than one attached drive to back up, rather than folding them into a Time Machine archive, a more robust third party backup tool is a good solution because it has a well defined procedure for recovering that secondary volume.

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Ashley Grayson

Thanks for this note. I’ve avoided the issue by simply not trying to back up a secondary hard disk in TM. I use Carbon Copy Cloner or simple Finder copies to backup external drives to other drives.

However, there’s a worse situation that can happen. If you occasionally make a clone of your boot drive with Carbon Copy Cloner or other software, always turn off Time Machine before starting the clone, otherwise, when you later boot from the clone (even just to verify it is valid), it will try to back up to Time Machine. If it does, it will poison the existing Time Machine archive because Time Machine thinks that the known computer, booted from a different drive is the same system but that it has never been backed up. Time Machine will add the entire clone to the old archive as all new files, not make a second archive of the clone system—which might be OK.

I think this is a flaw in Time Machine: it defines a “system” to be a CPU, not a bootable disk image and backs up to that “system archive” any disk that CPU is booted from. This is probably done so that deployments of identical software to multiple computers (like in a school lab or internet cafe) will be unique, because they are on different CPUs, but it doesn’t work if one user has alternate boot drives for a single Mac. Consider: if you have a Mac with 100GB of files on Macintosh HD, happily backed up on TM and you want to install a new OS update or make an emergency boot drive from the Install CDs, if Time Machine runs while booted from that alternate drive, it will assume you deleted all 100GB of files and make an updated archive of the new disk. All the old files will still be in the archive, but who wants such a mess. Time Machine and Time Capsule are woefully lacking in warnings and there is no documentation available, even to second tier AppleCare techs to warn of or explain how this aspect of TM works.


Wow, I had no idea that there was any issue with backing up additional drives. Like many people, I’m sure, I started up and configured Time Machine without much thought about how the restore process would work. I just thought I would read up on it when and if I had a failure of some sort.


You have been discussing using Time Machine to restore an entire drive due to failure.  Does Time Machine work normally with secondary storage drives to retrieve old/deleted single files?

What does Time Machine do with Boot Camp partitions?



This sad news does not surprise me. In 2006, I experimented with using the Backup 3 application that came with .mac membership. When my main drive unexpectedly failed, it was impossible to restore any of the files that had been backed up using Backup 3.  It turned out that, according to a .mac tech, when the directory of the disk that was backed up was damaged, the restore didn’t work properly. Of course this is precisely the situation in which backup software is supposed to work. What Backup 3 did was to provide an illusion of data safety rather than actual data safety. Luckily, I had used Backup 3 only as an experimental backup - - the main drive was backed up, in triplicate, to tape using Retrospect. Retrospect eventually restored all my files - - although there was one heart stopping moment when Retrospect announced, after several hours of catalog reconstruction, that the process had failed due to “tape error.” So I started again with the second backup set - - which worked. I never again used tape for backup. Nowadays hard drives are less expensive than the same amount of storage on tape.

As Mr. Martellaro suggests using “robust” backup software that provides actual data safety is essential.


You seem to imply that one can only restore the main boot volume using Time Machine. I am not sure my situation applies here. I have 4 internal drives on a quad Mac pro. One of these is dedicated to Time Machine only. Each of the other drives contain volumes such as the boot drive plus data volumes, plus backups to the boot and data volumes. The backups are generated via Carbon Copy Cloner.

I have setup TM to backup the boot volume plus the two data volumes.

Now, suppose I enter TM and want to restore one of the data volumes whose drive has failed. Are you saying that I cannot restore that volume to a new drive using TM? Also, suppose, something corrupted a data volume but its drive is fine. Can I restore the data volume back into its original drive overwriting the corrupted version?


Tom Casselman


You are correct that you cannot restore a boot volume via the full system restore option.

But you can easily restore a data volume via Time Machine’s “Star Wars” display.  See #E3 in Time Machine - Troubleshooting to see the backups of a disk that’s no longer connected.

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