$ sudo mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/rd0 --write-mostly /dev/sdc2
mdadm: /dev/rd0 appears to be part of a raid array:
       level=raid1 devices=2 ctime=Sat Jan  9 16:32:35 2021
mdadm: Note: this array has metadata at the start and
    may not be suitable as a boot device.  If you plan to
    store '/boot' on this device please ensure that
    your boot-loader understands md/v1.x metadata, or use
    --metadata=0.90
mdadm: /dev/sdc2 appears to be part of a raid array:
       level=raid1 devices=2 ctime=Sat Jan  9 16:32:35 2021
Continue creating array? y
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.