Create a new filesystem
First create two new identical partitions using fdisk or the likes. Toggle filesystem type to FD (Linux RAID Autodetect).
Then:
mdadm –create /dev/md6 –auto=md6 –level=raid1 –raid-devices=2 /dev/sd[ab]6
The array is now created and operational, but the config file /etc/mdadm/mdadm.conf needs to be updated:
mdadm –detail –scan
This will provide details of the array configuration in a format that can be used in mdadm.conf