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If the md device provides the file system including /boot or is an LVM PV (physical volume) then /etc/mdadm/mdadm.conf must be re-populated as shown below and the initramfs re-generated by:
update-initramfs -u
EFI System partition (ESP)
EFI only recognizes FAT16/FAT32, so according to most sources, RAID 1 is not possible. However, that is not true: RAID 1 is possible as long as metadata version 0.9 or 1.0 is used. The reason it works is because metadata 0.9 or 1.0 is stored at the end of the partition, which doesn't interfere with EFI, whereas metadata 1.2 does since because it's stored at the beginning.
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Back up the current ESP files and unmount the file system: cd /boot && tar -cvzf efi.tar efi umount /boot/efi
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