I moved my server from Mac mini to MBP Intel since I was using it also as a media server. Since then I've moved my music and videos to my MBP M4 and the MBP Intel has started doing some heavy processing around midnight for an hour or two. I researched and maybe it was Spotlight indexing Bootcamp partition but even with that excluded the task run hard when I was trying to sleep. So I decided to clean install macOS.

First I wanted to move the existing install to an external HD. Trying to install macOS on an external HD comes up with an error because Macs for several years now are by default setup to ignore booting from an external drive (security feature). You have to boot into Recovery Mode than run a utility to turn it off and with that utility you can also change OS verification — by default older unsigned macOS are not allowed to boot since they might be compromised without any security updates; that is if there is a more recent supported macOS that runs on your machine.

But on booting from external HD it kept crashing and rebooting by itself. Even using Carbon Copy Cloner 6 (which uses an Apple APFS utility) to clone to an external HD still had that problem.

I had totally missed that with APFS you can have multiple volumes on a device and each volume can have its own macOS install: Use more than one version of macOS on Mac. I created another volume, ran macOS installer and installed in that volume and everything worked. Mind you my MBP Intel has an 8 TB SSD so trivial to do. On my MBP M4 with a 512 GB SSD I might be able to pull it off (currently using 200 GB of my SSD). Anyway, you use Startup Disk (or reboot with Option key) to change to the other macOS.

Also the other thing is that booting macOS from an external HD and USB 2 connection is rather glacial. I can see why everybody says modern macOS requires an SSD.