Create A RAID-1 Array
mdadm --create /dev/md0 --level=1 /dev/sda /dev/sdb

Scan For Disks
mdadm --detail --scan

Add Disks To mdadm.conf
mdadm --detail --scan >> /etc/mdadm/mdadm.conf

Check Status
cat /proc/mdstat

Mark Disk as Failed
mdadm --manage /dev/md0 --remove /dev/sda

Remove Failed Drive
mdadm --manage /dev/md0 --fail /dev/sda

Add Disk To Array
mdadm --manage /dev/md0 --add /dev/sda