Setup

sudo apt install mdadm

Activate RAID5 on three disks

mdadm --create /dev/md0 --level=5 --raid-devices=3 /dev/sda1 /dev/sdb1 /dev/sdc1

mdadm state

cat /proc/mdstat
mdadm --detail /dev/md0

Check a RAID for its state

insert any of its devices

sudo mdadm -E /dev/sda1

Add it to the fstab

/dev/md/0    /mnt/raid_01    ext4    defaults    0   0

…or use its uuid…

And append it to the config…

sudo mdadm --detail --scan >> /etc/mdadm/mdadm.conf

Results is ARRAY /dev/md/0 metadata=1.2 name=server:0 UUID=xxxxxxxx:xxxxxxxx:xxxxxxxx:xxxxxxxx

Add a spare drive

mdadm --add /dev/md/0 /dev/sdd1

(Re-)Add an old member drive

mdadm --re-add /dev/md/0 /dev/sdd1

Remove an member drive

1. Mark it as failing (or it already has an (F) in mdstat)

   mdadm --fail /dev/md/0 /dev/sdd1
   

2. Remove it…

   mdadm --remove /dev/md/0 /dev/sdd1