Felix Schwarz

Just spent hours trying to install macOS 26b2 on an external SSD. After rebooting, it just wouldn't complete but boot back into macOS on the internal SSD eventually.

Why? I had plugged the SSD into the wrong USB-C port: on Apple Silicon Macs, you're supposed to NOT use the DFU port for this (https://support.apple.com/en-us/111336) - and of course I had. 🤦‍♂️

This document tells you which of your Mac's ports is the DFU port:
https://support.apple.com/en-us/120694

Avoid that port when installing macOS on a external drive.

Install macOS on an external storage device and use it as a startup disk - Apple Support

If you have an external storage device, you can set it up as a Mac startup disk.

Apple Support
Jun 25, 2025 at 6:18pmWeb
Show threadJochen MarschallJun 25, 2025
@felix_schwarz
Thank you!
I looked up my mini. Make sure you face the back when finding the middle port. 🤓
Show threadDer TeilweiseJun 26, 2025

@felix_schwarz The problem is not that Apple does not control the hardware. It’s not that the installer cannot know what port the external storage is connected to.

The problem is that Apple does not care to prevent the installer from breaking the users installation.

(BTW: As I understand it, you can use that port to boot, it’s just the installation that fails, right? I really would like to know why you cannot install macOS over that port.)

Show threadFelix SchwarzJun 26, 2025
@teilweise Couldn't have said it better. And yes, at least afaik, you're expected to be able to boot using that port once the OS is installed, just not use it for installation. Why that is, I don't know, but would also be eager to know.
Show threadcrypticcelery đź”™ EH23Jun 26, 2025
@felix_schwarz I did not know you could boot at all from external media on Apple silicon (though it might be a limitation of Asahi I read wrong).
Show threadFelix SchwarzJun 26, 2025
@crypticcelery IIRC support for this was buggy and/or bumpy in the early days of macOS 11, but it works relatively well these days. What's new is the concept of boot volume ownership, though, that will prevent you from installing macOS on an external drive on one machine, then take that drive, plug it into another Mac and boot from the drive there. Because the owner (stored on the internal drive) will differ. That's something I really miss from the Intel days.
Show threadJaanus KaseJun 30, 2025

@felix_schwarz @crypticcelery also what seems to be undocumented is that macOS 26 has restrictions when booted from external device

E.g Foundation Models APIs are not available in simulator or macOS, Xcode AI assist is not available

Show threadAndy ParkJun 26, 2025
@felix_schwarz ouch. No warning either? You’re just supposed to know this beforehand?
Show threadFelix SchwarzJun 26, 2025
@thebearmaster Yup, no warning unfortunately. I'd have expected the Installer to recognize this and either warn me or even prevent me from picking the volume for installation.
Show threadAndy ParkJun 26, 2025

@felix_schwarz right, I’d have expected that much.

It hurts as a long time Mac user to suspect that Apple is no longer the same company that used to pride itself in handling details like this.

Show threadeuroplus Jun 26, 2025
@felix_schwarz @arroz “Use USB”, they said, “No more SCSI Voodoo”, they said!