Add new bootable EFI disk to boot-pool mirror?

[whodat]

I have an EFI system with 2 disks in the boot-pool mirror, configured with swap:

Device Start End Sectors Size Type /dev/sda1 4096 6143 2048 1M BIOS boot /dev/sda2 6144 1054719 1048576 512M EFI System /dev/sda3 34609152 168826879 134217728 64G Solaris /usr & Apple ZFS /dev/sda4 1054720 34609151 33554432 16G Linux swap Device Start End Sectors Size Type /dev/sdb1 4096 6143 2048 1M BIOS boot /dev/sdb2 6144 1054719 1048576 512M EFI System /dev/sdb3 34609152 168826879 134217728 64G Solaris /usr & Apple ZFS /dev/sdb4 1054720 34609151 33554432 16G Linux swap

What is the "correct" way to attach a new 3rd disk to the boot-pool mirror, so the new disk is also configured as EFI bootable, and partitioned for swap i.e. so that if 1 or 2 disks fail, any remaining disk could be used for boot?

I realise I can download my TrueNAS SCALE config and re-install with all 3 disks as the boot-pool, then re-apply the config... but I would like to know if there is another way, ideally via the GUI. I was hoping to find something in System Settings > Boot > Actions > Boot Pool Status , but had no luck.

If not via the GUI, can this be done via the CLI? This post mentions dd'ing the boot and EFI partitions, and attaching to the pool, but it isn't clear on how to make the drive bootable in EFI:
https://www.truenas.com/community/t...sds-with-boot-and-data-pools-for-scale.89428/

 

[diogen]

What is the "correct" way to attach a new 3rd disk to the boot-pool mirror...

I think the "correct" way is to re-install TrueNAS from scratch on three disks.
SCALE is quite flexible and you should not lose any information...

...it isn't clear on how to make the drive bootable in EFI:

My understanding is, copying the boot-pool is making the disk bootable...