Technical Analysis: Time Machine Restore of Secondary Volumes

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.