First get information about the raid array: cat /proc/mdstat Then get information about the drive you’re going to add fdisk -l /dev/DISK_TO_ADD If there are any partitions, delete them and create a one new primary partition with type fd – Auto RAID Add the prepared drive to the array: mdadm --add /dev/RAID /dev/DISK_TO_ADD And expand it: mdadm --grow /dev/RAID --raid-devices= N+1 # N - the number of drives in the raid array before the expansion Seat comfortable and watch the raid being rebuilded: ...