I find many answers to this question for Ubuntu 12 and 14, but with EFI partitions things have changed a bit. Actually using those guides I do have an older system running RAID1 but I have run into trouble, well speed bumps.
The older RAID system had my backup and nextcloud data, which I have moved to the new system, recreated raid and copied.
This is what I have running right now:
Device Start End Sectors Size Type
/dev/sda1 2048 1050623 1048576 512M EFI System
/dev/sda2 1050624 959449087 958398464 457G Linux RAID
/dev/sda3 959449088 7814035455 6854586368 3.2T Linux RAID
Device Start End Sectors Size Type
/dev/sdb1 2048 1050623 1048576 512M EFI System
/dev/sdb2 1050624 959449087 958398464 457G Linux RAID
/dev/sdb3 959449088 7814035455 6854586368 3.2T Linux RAID
Device Start End Sectors Size Type
/dev/sdc1 2048 1050623 1048576 512M EFI System
/dev/sdc2 1050624 976771071 975720448 465.3G Linux filesystem
The system currently boots from /dev/sdc
I have tried a few things with a Live CD to copy sdc1 and sdc2 to sda and sdb and make them bootable.
I found this post quiet interesting Install Ubuntu 20.04 desktop with RAID 1 and LVM on machine with UEFI BIOS
I think it has many of the details I have been missing.
Specifically step 4.1 chroot to the raid file system I have mdadm installed and configured so all that is left is
Update the module list the kernel should load at boot.
echo raid1 >> /etc/modules
Update the boot ramdisk
update-initramfs -u
Finally, exit from chroot
and then continue with steps 5 onward.
I honestly think the key point I was missing was adding raid1 to /etc/modules In the older docs (Ubuntu 12 etc) I think it was handled in the grub files
