Running the current macOS from an external boot drive

> Will Applying the Update of Big Sur directly to existing Legacy Bootable Clone achieve and updated Bootable Legacy Clone ?

Yes (if I understood the question correctly): it is possible to boot to the legacy clone and update its Big Sur us usual.

Or: apply the full Big Sur installer to the legacy clone while booted from the source or from a flash installer etc. This way it is also possible to start making the default data-only Big Sur clones and only later make it bootable by applying the full Big Sur installer to that volume.

One thing of note on M1 Macs, the internal recovery volume will be what ever the latest OS version that has been installed, regardless if it is installed internal or external. Example, installed production macOS Y on internal drive. Then install macOS Z on the external. The internal recovery will now contain the Z recovery and attempting to recover the internal Y install will only get offered the Z OS to recover. I have verified this when I was running Monterey Beta on my external Samsung T7 SSD.

Also note, on M1 Macs, the recovery/firmware partition is not cloned and it is necessary to boot an M1 Mac. So, If the internal drive on the M1 Mac dies, with that boot partition gone, you cannot boot any drive, period. Also, because of that "special" partition on M1 Macs, you cannot clone an external clone back to the internal.

So, your recovery options on an M1 Mac should the system get corrupted:

1. Assuming the recovery portion is still good, wipe and re-install macOS and then either restore data from Time Machine backup or use Migration Assistant with the clone to restore apps and data.
2. Use Apple Configurator 2 to Restore macOS and use the previously mentioned methods to restore apps and data.

If that boot partition on M1 Macs gets corrupted, assuming the drive is still good, it can be "Revived" using Apple Configurator 2 without wiping the system.

BTW, Time Machine, at least on M1 Macs, does not do a full system backup. It only backs up the Macintosh-Data drive and therefore can no longer restore a complete system (Big Sur forward).

Unfortunately in the cause of security, Apple has made something that was quite simple and made it difficult.

Carbon Copy Cloner can make a bootable Big Sur clone to an external drive (AFAIK Super Duper can now do that as well).

The only new negative issue is that the target volume must now be erased in the process. After that the user can do the default data-only clones to the external volume.

If the source volume's Big Sur is updated, then the target volume remains in the older version. The user might be able to continue booting from the external volume after that. But in my experience sometimes (maybe if there is enough difference in the Big Sur versions?) the target volume then no longer successfully boots. CCC author is aware of the issue and suspected it is the result of changes to a handful of Apple system files that reside on the Data volume and that these get out of sync with the older OS on the destination and that this causes trouble for booting from that volume.

Workaround if you want to keep the clone bootable: a) boot to it and update also its Big Sur or b) if it no longer boots, apply the same version Big Sur full installer to the clone either while booted from the source volume or from a USB Big Sur flash installer.

https://bombich.com/kb/ccc6/cloning-macos-system-volumes-apple-software-restore#caveats

AFAIK silicon Macs should behave about the same but there are reports that a Thunderbolt connection might be required for external booting?

Creating legacy bootable backups of macOS Big Sur

Will Applying the Update of Big Sur directly to existing Legacy Bootable Clone achieve and updated Bootable Legacy Clone ??

Thanks.

Wanted first hand experience and you have supplied :-))