$ 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.