What exactly are you trying to accomplish? It does not matter if your boot volume is internal or external. You can still restore from Time Machine.
I'm trying to accomplish an impossible thing:
Running a basic desktop Mac free of apple tax of soldered ssd storage.
(Doomed to failure.)
I originally thought that everything is like years before.
You boot from
whatever disk and then be happy with it.
But, "new rules of Apple" tells me, that you have to have your homedir in startup drive, or you'll never gonna be able to fix permission issues, if they occur:
https://support.apple.com/en-us/HT203538
And believe me, running macOS from internal ssd, when homedir is in external ssd, will lead to a BIG problem.
I'm trying to find out if it's about local corrupted TM backups or what, but TM fails to have full backups and Mail doesn't show attachments of my IMAP mails (for 25 years now..).
I have checked that almost none of my mails in past 4 months are backed up by TM.
You can't fix that with Apple's DU, you can't clone the "corrupted" external drive with Apple's DU.
Or any APFS startup drive to another, so the clone remains bootable.
I'm just researching if CCC is helping (need to pay for upgrade).
What makes me angry, is that Apple hasn't documented the new restrictions.
When you move homedir in Prefences, there's now warning that moving homer to some other drive than startup drive, will lead to that, you can't repair homer's permissions.
Also, you can happily back up your system to TM, but it doesn't warn you, that you can't restore your system to external startup drive:
https://discussions.apple.com/thread/8417393
And of course, there's no mention that if you do the "illegal thing", put your homedir in another drive than startup drive, your TM backups are not working.
Also, you can't clone your APFS startup drive to external drive, to have that new external drive to be bootable.
At least with Apple's Disk Utility.
It took 6 hours form my new mini to clone startup drive to another drive.
I checked from Activity Monitor that it took over 3TB to read and 3TB to write for 550GB of data from a startup drive to another drive. And it ended to an error.
I've also tried to fix the permissions, or whatever, with supported, but undocumented terminal command or with Disk Utility's First Aid, but after 24 hours it was still not done and almost all of the time DU is "not responding". This is for the external 1TB ssd (with speeds that exceeds 100MB/S) and what was first for just homedir and then, when I was told that's "illegal", I installed the whole OS to it. So that the homedir is in same drive that is the startup drive, which is now the requirement for any support from Apple.
All this might be because of local corrupted TM backups in my external startup drive (1TB ssd through usb 3.1 (10Gb/s link)), but again, APFS is so undocumented and outside of "normal supported environment" (just booting from soldered internal drive), that Apple's Support gives no help whatsoever.
Really nice to have this new secret undocumented security things like SIP, T2 and APFS.
Apple has gone far from the original "open source" ideology what Mac OS X was...
Btw, when Apple stopped updating their man pages in web?
PS. I asked for support, if I could change my mini to one with bigger internal ssd, even with juicy apple tax on internal ssd, and they, of course, said, not possible.