Install Sequoia to USB drive and keep Tahoe on internal

[gklka]

For development and testing purposes I have to have a macOS Sequoia instance with Xcode being able to build locally (to macOS target). For this I wanted to install Sequoia into an USB drive attached to my M1 MBP. I am not new to OS installs, done this since 23 years now, but this one got me, so I ask for help.

What I did: So the machine has Tahoe installed and I don't want to touch that. I got two USB drives, one for the install image and one for the installed Sequoia system. Downloaded 15.7.2 installer via Mist, used createinstallmedia command to create a Sequoia installer into one of the drives, booted from it and started the setup for the other USB drive. It goes almost for a hour, gets until around 60%, stays there for a lot of time, then quickly gets 100%, and reboots to the preinstall system. ...which doesn't work. I always end up in the main Tahoe system. Neither in Tahoe or in recovery Startup Disk preferences cannot select the USB drive to boot, because "it has a macOS version which needs to be reinstalled". In the boot menu (which you can access by pressing power on long) I can see the drive but pressing continue below it just does nothing. I also tried building the installer directly to the drive with Mist. Same results.

I also tried to reduce security for the internal drive, to see if that helps. I've read that I probably need to reduce security of the external drive, but to do that, it needs to be startup drive first, which I cannot set.

So here I am. Any ideas what did I wrong? Is the current 15.7.2 installer just broken and doesn't support installing it as a secondary system to USB? Or what?

Any help appreciated.

 

[Bigwaff]

Make sure your external boot drive is not connected to the USB-C DFU port when you install macOS to it.

 

[gklka]

Make sure your external boot drive is not connected to the USB-C DFU port when you install macOS to it.

Which one is that?

 

[genexx]

And you have to use a Thunderbolt Cable! Even with a USB3 1GB Connected Drive and as you should know, Mac is picky with Drives.

In addition you have to install a fresh OS and can´t boot from a clone.

The Best is to use a Acasis TBU405 AIR (Cheaper) and a WD SN 570 / 770 / 850 / x Drive also the new 7100 are working.
You can use other, but they mostly do not work with the Power saving features and are getting hot while sucking Watts.
The Acasis TBU405 AIR with the WD SN 850x works like the internal Problems.
I Test the Betas like this.
Only Apple Intelligence does not work if you boot from Externals.
For Coding i am also still on Sequoia with my MacMini.

How to identify the DFU port on Mac - Apple Support

Learn how to find the device firmware update port on your Mac.

support.apple.com

DFU port locations​

With some exceptions, the DFU port is the type USB-C port farthest to the left (leftmost) or farthest to the right (rightmost) when you’re facing the ports. Some examples are pictured below the table.

Mac		DFU port location
MacBook Pro	Models with Apple silicon	14-inch MacBook Pro with M4 or M5 chip: The rightmost USB-C port when you’re facing the left side of the Mac All other models: The leftmost USB-C port when you’re facing the left side of the Mac