Why am I unable to boot macOS from an external device on MacBook Pro M3?

Question

I’m trying to install macOS 15.0 (24A335) on an external memory device and boot from it on my MacBook Pro with the Apple Silicon M3 chip that runs the same build on the internal SSD. Here’s what I’ve done so far:

1. Initialized the external device with a GUID partition map
2. Created an APFS partition on the device
3. Downloaded the macOS 15.0 (24A335) installer directly from Apple's SWCDN
4. Ran the InstallAssistant.pkg and obtained a copy of Install macOS
5. Launched Install macOS and installed the OS on the external device

Everything went smoothly up to this point. However, after the installation:

• I tried selecting my external device as the startup disk and restarted the computer, but I received the error message "The version of macOS on the selected disk needs to be reinstalled."
• I booted into recovery mode, and my external device appeared in the list of bootable disks. But when I selected it to boot, after some initial loading, nothing happened.
• I also selected the device as the startup disk from recovery mode, but again got the same "The version of macOS on the selected disk needs to be reinstalled."
• I made sure that my Startup Security Utility settings are set to "Reduced Security" and that the device's information on Disk Utility shows "bootable: yes"

I’ve previously installed and booted countless operating systems on my older Macs and PCs with no issues, from unsupported macOSs to Windows and Linux distros, thanks to the abundance of forums and guides online. This time, I’m struggling to find anyone with a similar issue, and there’s very little information available. One website briefly mentions that "It's not as easy as it used to be, as it likely requires you purchase new hardware", but it seems to refer to needing a USB 3.1 or higher external device, which I tested with a USB 3.2 flash drive without success. Another blog even provides a tutorial on "How to Boot Intel/T2/M1/M2/M3 Mac from External Drive", which suggests this should be possible, but I haven’t had any luck. I read some other things around, such as naming the device with no spaces and changing the USB port to plug it into, but nothing worked.

I’d prefer to boot macOS from an SD card in the built-in SD reader (for minimal clutter, even if it'll be slow), but I’m open to buying a new USB or Thunderbolt device if it guarantees success.

Has anyone encountered a similar problem? Any suggestions on how to resolve this?

Answer 1

Rather than make a long chain of comments, here are reasons why I would recommend you start over with instructions purely appropriate for macOS external boot on Apple silicon. I am assuming you are not trying to create a bootable installer.

1. You do not need to change boot security - any guide asking for that is likely wrong. The only exception is if you intend to tamper with macOS on the external and not just run the installer Apple provides.

2. No Apple Silicon hardware boots directly from an external source. They all boot to the internal disk and then once the local security policy is set up with the Secure Enclave, continue booting from the selected OS which could be an external drive if you complete the install process. This means you have users on the internal drive, OS on the internal drive and a second OS on external drive that can’t ever boot if there’s a problem with the internal drive or users.

   LocalPolicy is specific to a System Volume Group and authorized user.

3. If you asked how to make your Mac as slow as possible, setting up an OS on the SD card is the ticket. It’s ok if you have reasons to attempt this, but it’s going to be the most painful and least performant choice you could make with this M3 hardware. I would move directly to a full Thunderbolt 3 or 4 drive before attempting this myself.

4. Start with Howard Oakley eclectic light:

   • https://eclecticlight.co/2022/04/29/which-ssds-can-you-boot-your-m1-mac-from-do-hard-disks-work-too/

   The above article links to excellent (and not so much) performing external drives for M1 and newer Macs and covers external booting with this recommendation.

Don’t do it

And since he did do it, and wrote about it, we all can learn a lot and save mis-steps:

• https://eclecticlight.co/2023/11/09/should-you-make-an-emergency-disk-for-an-apple-silicon-mac/
• https://eclecticlight.co/2022/11/12/what-makes-an-external-disk-bootable/ (absorb the T2 and Intel details as they set the stage, but ignore them for M3 use - your hardware is much more constrained than Intel with respect to external booting)

Good luck starting down this path anew if that’s your decision. I would erase the SD using disk utility and download the installer from the Mac App Store and run it. That replaces your steps 1-6 with much better chance of success (especially if you use a fast external SSD with USB 4 or Thunderbolt 3 class drive interface).

Answer 2

• Do I know why did the problem occur? Nope.
• Did I manage to do what I wanted? Yes.
• Am I providing a solution? Maybe.

Since many resources online suggested using an SSD (mainly for speed, I guess), I decided to give it a try and bought an M.2 SSD + NVMe enclosure, so that's the hardware I ultimately used. After tens of attempts with something changed every time, a question flashed through my mind:

• Could I have tampered with the computer with the couple of changes I have done since I got it?

So I tried doing this:

• I created another user on my Mac and repeated the procedure.

So basically I repeated the same steps as usual while logged into my Mac as the new user. I downloaded the macOS installer from the App Store (but I'm not sure this makes any difference from using the SWCDN pkg - which I believe to be just the same) and installed it onto the APFS-formatted SSD setting that new user as owner. Et voilà! Now it works as it should!

For the sake of completeness: I tried doing the same on my USB 3.2 flash drive and the SD card with no luck. Also, I used macOS 15.0.1, which had been released in the meantime, but made many unsuccessful attempts with it too, so I don't think this is decisive. I'll update this answer if I try anything new.

I'm perfectly aware that this answer does not give a shred of explanation, but switching user worked for me somehow, and it could be helpful in case someone else encounters the same issue!