Booting two Apple silicon Macs from one external disk

I’m only too aware of how many of you want to be able to boot an Apple silicon Mac from an external disk. I also know that a significant few want to do this routinely, and even boot more than one Mac from the same external disk. This article takes you through a demonstration of how to do that in macOS Ventura, and adds updating macOS to the challenge.

What I did was:

  1. Using my Mac Studio Max, I installed Ventura 13.0 on an external SSD, and confirmed that could be used to boot that Mac.
  2. I then moved the bootable SSD to my MacBook Pro M1 Pro, and booted it from that external disk.
  3. When booted from that external disk, I updated macOS on it to 13.0.1, using Software Update.
  4. I then moved the bootable SSD, now running 13.0.1, back to my Mac Studio, and booted that from it.

Principles

Traditional use of an external boot disk on multiple Intel Macs has normally booted those Macs only from the external disk. While that may well be feasible with Apple silicon Macs, they’re designed primarily to boot from their internal SSD. Maintaining an updated bootable macOS on the internal disk is important, as it ensures that Mac’s firmware (including LLB, the first stage bootloader) is up to date, and that there’s a full recovery system available even when the external disk isn’t connected, or has a problem.

Throughout this, the internal SSD in each Mac remained at macOS 13.0.1, and should be updated before updating that on the external SSD. Both Macs were shut down after they had been booted back to the system on their internal SSD. Although this makes use of the external bootable disk a bit more ponderous, I believe that it’s safer and more reliable.

Thus, the sequence for using each Mac was:

  1. Connect the external disk and boot that Mac from its internal SSD.
  2. Open System Settings > General > Startup Disk, select the external disk, and restart.
  3. On completion, change the startup disk back to the internal SSD, and restart.
  4. Shut that Mac down, so that on next starting up, it will boot from its internal SSD.

However, if you’d prefer to leave both Macs expecting to boot from the external disk, macOS will accommodate that perfectly well too. Should you inadvertently start up either Mac when the external disk isn’t connected or can’t be recognised as the boot disk, then after a long pause while that Mac tries to find the missing disk, it simply restarts in Recovery mode, giving you the chance to rectify the situation.

Install macOS on the external disk

I’ve already described this in detail. Key steps set Ownership of the external disk, so that its LocalPolicy can be set and saved.

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Boot the second Mac from the external disk

Ownership and LocalPolicy are set after selecting the external disk as the startup disk, as described here.

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Updating macOS

This is performed conventionally, using System Settings > General > Software Update. If that doesn’t work, then restart the Mac (from its external disk). If that still fails, start it up in Recovery mode, select the external disk, hold the Shift key and click on the button to start it up in Safe mode, and try from there.

Boot the first Mac from the external disk

When macOS has been updated on the external disk, LocalPolicy has to be updated to reflect the new version of macOS. After selecting the external disk as the startup disk, there may be a long pause while controls in that System Setting appear unresponsive. Eventually, it should be possible to click on the Restart button to reboot that Mac from the external disk.

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Apple ID and iCloud

If FileVault is enabled on the internal SSD, the user will be prompted to enter its password shortly after starting up from the external disk. Although a minor nuisance, this is hardly onerous, and inevitable.

What is a potentially greater pain is having to sign into your Apple ID and iCloud after starting up from the external disk. Accordingly, during my testing I don’t configure Apple ID or iCloud for external boot disks. However, those processes are independent of the architecture of Mac being used.