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mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb1 /dev/sdc1
In case only one underlying device is available (for example during system build or recovery) the same command can be used with the missing underlying devce name replaced by "missing". For example:
mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb1 missing
If the md device provides the file system including /boot or is an LVM PV (physical volume) then /etc/mdadm/mdadm.conf and the initrd must be updated
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