Can we add a 3rd mirror to boot-pool?

Yes, I thought you could use the native Linux partition tools and then dd(1) to copy over the GRUB partition from your current drive A.

What do you mean by “compare A and C”? You need to construct C as a copy of A as far as partitions are concerned.

If you suspect there might be something wrong, why not set a convenient maintenance window, export and save config, reinstall, import config?

The boot pool layout, GRUB and stuff are not part of the saved config. You will be getting a clean factory new boot pool and auxiliary partitions.

P.S. Just noticed the other thread.

See Clean install of 24.10.0 results in "cannot import ‘boot-pool’ : no such pool available" - #31 by pmh