Felix SchwarzJun 25, 2025

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
Show threadcrypticcelery 🔙 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 Schwarz
@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.
Jun 26, 2025 at 12:46pmWeb
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