Beyond Time Machine: 2 Infrequent backups

When I evaluated what I needed in my backups, I realised that making full backups of everything on my startup Volume Group and an external working volume was simple but highly inefficient. Looking at those needs, I drew up a list of five requirements:

  1. a non-bootable copy of the System volume after each update to macOS, for reference;
  2. a full copy of my Data volume and its external supplement, updated nightly;
  3. a full copy of my ~/Documents folder updated hourly;
  4. a full copy of my main working folder on an external SSD updated hourly;
  5. a Time Machine backup of that same working folder every hour.

Being smaller, these enable me to use the four SSDs in my ThunderBay enclosure as individual disks, rather than requiring a RAID array. My next task was to decide which backup utilities to use for each of those. This article considers the first two on that list, backups which are made once a day or less frequently.

1. System volume copy

The most common reason for me to access my old Time Machine backups was to dig back through old versions of macOS, to find previous releases of its tools and other system files. This is perhaps inevitable given my interest in macOS, and I accept that it’s an unusual habit. What I therefore want to keep is a copy of the System volume for each updated version of macOS, which don’t have to be bootable. This isn’t as straightforward as it might seem.

I have both the traditional and long-proven utilities Carbon Copy Cloner and SuperDuper!, which originated as volume duplicators, so should be ideal for this task. There are two slight snags, in that they will only clone Macintosh HD, my startup Volume Group, to a whole volume, and don’t readily distinguish between the contents of the System and Data volumes. In the end, I settled for Mike Bombich’s Carbon Copy Cloner, but I’m sure you could do this just as well with Dave Nanian’s SuperDuper!

Key to understanding how to do this is remembering that volumes in APFS are very different beasts from those in HFS+. I originally wanted to copy my System volume into a folder rather than a volume, and was irked that I couldn’t. In APFS, volumes behave in many respects like folders, in that they share the same disk space and aren’t ‘partitioned’ from one another. So all I did was create a new APFS volume named according to the system version I was going to clone there. If you don’t want this cluttering up the Finder’s sidebar, which is now getting quite full, eject that volume alone and leave it unmounted until you need it.

The next snag is that, like the Finder, these cloning utilities don’t distinguish what’s on the System and Data volumes. For this I referred to my Catalina volume layout charts, and removed those items like /System/Volumes/Data which aren’t on the System volume. I did, though, keep some of the writable system files which are stored there.

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This is a one-time backup which I’ll have to set up for each fresh update to Catalina, and the whole lot came to less than 40 GB when I made that copy. When I have updated to 10.15.4 I’ll then repeat the process, creating a new volume on that disk with a new one-time cloning task to create it. A potentially useful side-effect is that these should also be bootable, although that isn’t part of my disaster recovery plan, which starts in Recovery mode with a fresh install of macOS from Apple. It could be a useful option.

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2. Data and external volume copies

The second backup need is a daily complete copy of my boot Data volume and external working volume, another straightforward job for either Carbon Copy Cloner or SuperDuper!, although because the contents to be copied are in two separate locations, this has to be done using two backup tasks. In the case of the Data volume, I chose it from its mount point on Macintosh HD, /System/Volumes/Data. There will be a little overlap with what is saved in the System volume copy, but that’s another bonus.

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These are intended to be my main backups in the event that I have to restore my entire system, but rather than pecking away at them every hour, they’re scheduled to run in the small hours each morning, when even I shouldn’t be using my Mac. I time them to run at 0315 and 0415, with CCC’s SafetyNet turned off, so that they represent just the last state of the two volumes, without any history.

The other big bonus which swings me in favour of using CCC for these backups are snapshots. While the backups themselves are only daily copies, enabling snapshots on important volumes gives the ability to reach further back than that latest backup, in case there’s something amiss or missing. CCC gives the best and most explicit control over snapshots of any utility that I’ve seen.

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I have turned CCC snapshots on for both the volumes that CCC is making daily backups, and set a retention policy that aims to achieve safety, in keeping older snapshots, without requiring excessive storage space for them. When you first start making backups, you’ll need to keep an eye on the space taken up by snapshots. If you’re not careful they can spiral out of control and swallow hundreds of GB of disk space. CCC’s defaults are usually quite close to optimum, and are easily controlled in its main window. If only Time Machine offered this.

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Having put this backup scheme to work, the next job is to keep a close eye on any errors, problems, and changing free space on the backup volume. This is easy to do in CCC’s Task History.

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Having met the first two of my backup needs using Carbon Copy Cloner, in the next article I’ll move on to look at finer-grained backups, including Time Machine, which cover the more active folders on my storage.