Last Week on My Mac: Do you still need that external bootable disk?

Now we’re into October, all eyes are on Ventura, promised for release in the next four weeks, assuming Apple doesn’t make it a special Hallowe’en edition. One question that has arisen is how best to make an external bootable disk in preparation. If you can’t reliably clone your current system, what options are there?

Cloning Mac systems didn’t really start in earnest until the release of Mac OS X. Prior to that, it was usually easy enough to copy the System and other supporting files when necessary. The first version of Mike Bombich’s Carbon Copy Cloner in January 2002, for Mac OS X 10.1 Puma, changed that. Long before Apple came up with Recovery Mode in Mac OS X Lion 10.7 in July 2011, there were only two practical options for tackling serious problems: single-user mode (SUM), or another bootable disk.

Those with Macs with multiple internal disk bays, like the Mac Pro and its predecessors, frequently cloned volumes and whole disks. Because those were rotating hard disks and vulnerable to fragmentation, one simple way to defragment all your files and free space was to clone a volume from one disk to another, and back again. That had the added bonus that you could then use the intermediate disk in an emergency, when your regular boot disk suffered problems.

From Lion to Sierra, Recovery Mode was a bonus, but because Recovery depended on a hidden HFS+ volume on the same disk, it was vulnerable to many of the same problems as the system volume on that disk. Apple provided Remote Recovery, but that was so ponderously slow that it couldn’t replace the second bootable disk.

From High Sierra through to Mojave, changes brought by the introduction of APFS made the second boot disk just as attractive, but Catalina went all complicated, with its more elaborate volume layout. With Big Sur, cloning all but ground to a halt: first, the only method to perform that, asr, suffered a serious bug and didn’t work, then it became clear that the complexities of the boot volume group made cloning inherently less reliable.

At the same time, Recovery Mode improved considerably, as it had to for Apple silicon Macs. The latter have to start their boot process from their internal SSD, even when starting up from an external disk. Initially Apple put the M1’s primary Recovery system in a hidden container on the internal storage, then in Monterey it moved to another volume in that boot volume group, with the hidden container used for fallback, in place of Remote Recovery.

Assuming you’re able to clone your Mac’s internal storage to act as an external bootable volume, how could you use that instead of Recovery Mode?

  • On Intel Macs without a T2 chip, it can be used as normal, as a complete substitute for bootable internal storage.
  • On Macs with a T2 chip, it can only be used if you have already downgraded that Mac’s startup security to allow it to boot from an external disk. Unless that’s done before the problem occurs, the external bootable disk is useless.
  • Apple silicon Macs don’t need their startup security to be changed to allow them to boot from an external disk, but if damage to internal storage is sufficiently severe to make Recovery Mode unavailable, they’re unlikely to be able to get far enough into the boot process to start up from an external disk.

There are two other good reasons for wanting to build a cloned external bootable volume.

One is to add your own tools to supplement those provided in Recovery Mode, for example a copy of Disk Warrior to enable rebuilding the directory structures of an HFS+ volume, or TechTool Pro for its hardware tests and other tools. Apple has improved those available in Recovery, and the most important third-party disk utilities simply aren’t available for APFS, as Apple still hasn’t provided developers with sufficient information to enable their development.

The other is to provide a fallback macOS known to work, in the event that the upgraded macOS has problems that can’t be solved. These most commonly arose as the result of defective or failed upgrades, which have largely been addressed by Apple’s new updater/installer and the use of the Signed System Volume (SSV), which verifies every last bit in the System volume is correct. There will always be users who want to have a fallback bootable disk, but that can easily be created by a normal macOS install, rather than requiring any form of cloning.

Over the last few years, changes in macOS have made cloning boot volumes increasingly difficult and unreliable, but at the same time the value in doing so has diminished. In the past, travelling for any length of time really required two external disks, one for backups and the other for recovery. With Apple silicon Macs, that second disk is now largely redundant, and if you can guarantee good access to a cloud backup service, you may even get away without the first. That really is a big step forward.