mdadm

https://www.tecmint.com/manage-software-raid-devices-in-linux-with-mdadm/

cat /proc/mdstat
cat /etc/mdadm.conf
sudo mdadm --query /dev/md127
sudo mdadm --detail /dev/md127
sudo mdadm --examine /dev/sdb1
sudo mdadm --manage /dev/md127 --re-add /dev/sdb1 # if using same disk
sudo mdadm --manage /dev/md127 --add /dev/sdb1 # if using new disk

• had to run this when one of the disks was offline on boot and the system dropped into maintenance mode

  mdadm --stop /dev/md127
  mdadm --assemble --scan