Boot from external hard drive not working

Peter Roussak wrote:

As I understand what you're saying, I shouldn't really worry about this anyway because as I get rid of my old Intel Mac and start using only Apple Silicon ones, the clones most likely wouldn't work and I should rely exclusively on Time Machine?

Read the CCC article I linked previously as I believe it contains the new preferred method of keeping a second bootable macOS drive available with the latest data files which may help get you by until you can reinstall the OS and restore from a TM backup (I also included another slightly different CCC article down below). TM should be a good option to have since it is Apple's own preferred solution, but TM does have some limitations, so having a second backup option or even a temporary replacement can be handy as well. Most people who value their data usually rely on more than one set of backups and many times will use two completely different methods of creating the backups.

Personally I always prefer to have some sort of spare bootable drive handy if for no other reason than for testing and attempting to repair the main drive or to possibly recover data that had yet to be backed up as the failure occurred before the next scheduled backup. A spare full macOS bootable drive gives you so many more options such as being able to download & create a bootable macOS USB installer (assuming the Mac is Ok except for the OS or internal drive) and to be able to confirm only the internal OS and/or boot drive is the only problem.

Unfortunately, due to Apple's infinite short-sightedness, even a spare bootable USB drive may not help with an M1/M2 Mac with Apple Silicon if the internal SSD fails or if the internal SSD is fully erased, because the Apple Silicon Mac's internal SSD contains some required system files which are necessary for booting the Mac even from external media. This has never been the case with any previous Macs (Intel or PPC). If the internal SSD fails on an Apple Silicon Mac, then you have a brick since booting from USB won't work without a working internal SSD which is soldered to the Logic Board.

https://bombich.com/blog/2021/05/19/beyond-bootable-backups-adapting-recovery-strategies-evolving-platform

https://tidbits.com/2021/05/27/an-m1-mac-cant-boot-from-an-external-drive-if-its-internal-drive-is-dead/

https://eclecticlight.co/2021/12/17/when-you-cant-restart-an-m1-mac-from-an-external-disk/

If it were me I'd erase the drive, reinstall the OS onto the external drive and then migrate my data back to the newly created drive.

That process is a whole lot simpler than trying to figure out what Seagate's problem is.

Bootable macOS clones are becoming a thing of the past due to all the latest security features Apple has implemented into macOS especially with Big Sur and Monterey. The developer of Carbon Copy Cloner (CCC) has already mentioned that bootable clones is now no longer guaranteed and is just a "best effort". Here is one post by the CCC developer about these issues and the recommended method going forward (clean install of macOS followed by using CCC or SuperDuper to copy the home user folder/files):

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

There have already been multiple reports on these forums where users have reported no longer being able to make a bootable clone of an M1/M2 Mac with Apple Silicon after a recent Monterey update (they claimed it worked previously before one of the updates).

I've never successfully used SuperDuper to make a bootable clone, but with CCC there are a few things to do such as making sure the destination drive is erased first. Both SuperDuper and CCC utilize the "asr" feature of macOS to make the clones.