Part of the reason that this recover seems to have gone well is that the system booted up and gave me access to everything.
How would I have been able to complete the recovery if it had been the boot device that had failed?
From what I understand, and from past experience of when software RAID1 setups have failed, it isn't possible to boot using the second drive.
Ideally, in that situation I would want to make the second drive the first drive, add a new blank drive, boot and the complete the exercise above.
What do I need to do to make the second drive bootable?
Both sda and sdb are currently identical. / is /dev/md125 which is sda3 and sdb3 /boot is /dev/md126 which is sda4 and sdb4 /boot/efi is only on /dev/sda1
Would copying /dev/sda1 to /dev/sdb1 using dd be enough to make the second drive bootable if the first drive fails?