Two Macintosh HD backups in one APFS Container

[BudSimrin]

I have recently upgraded from Mojave to Big Sur. I erased and partitioned an external HD to have one container with 7 volumes. 5 of the volumes are SD backups of various data disks, but 2 of them are backups of MacintoshHD. In both the Startup Disk preference pane and when I do an option boot, only one of the two MacintoshHD backups appear.

In the Shirt Pocket best practices it says "format the backup drive as APFS, and use APFS's very flexible "volumes" as your backup destinations, one per source volume". I guess I have two volumes for a single source. Is that the reason only one of them boots?

Can I fix this problem by creating a 2nd container (on the same external HD) and putting the 2nd backup there?

[dnanian]

Are you sure you made an erase-then-copy backup of both? Sounds like one may be a Smart Update.

[BudSimrin]

Oh. I missed that. Thank you. I am still using Smart Update.

I'll change all my SD backups to erase-then-copy.

What about the data disks? Is smart update still OK for those?

[dnanian]

Erase-then-copy doesn't have to be used for every copy. Only when you want to copy or update the OS. See this post:

https://www.shirtpocket.com/blog/ind...ot_a_marathon/

[BudSimrin]

So, if I have one-time erased-then-copied, unless there is a system update, I can then smart copy.

Except... if I want to create a schedule and then forget about it, I would be best off erasing-then-copying my MacintoshHD every time.

Feature request? Could SD check if a given backup requires system files, and if there has been a system update,then SD would choose the appropriate backup? I am thinking that Apple must leave itself a message somewhere of the system status.

[dnanian]

No, you'd be better off scheduling a Smart Update. You can always add a system or just restore the data, but if a drive fails during an erase, you'd lose the backup.

And that's why we don't do what you're suggesting. Unlike Catalina and earlier, we cannot update "just" the system without wiping the entire backup. Doing so is more dangerous than NOT doing so.