Create new primary partition
fdisk /dev/sdf
Set partition to Linux RAID Autodetect which is fd
sudo mdadm --add /dev/md0 /dev/sdf1
This will task appoximately forever.
sudo mdadm --grow /dev/md0 --raid-devices=5 # (amount of disks before + 1)
Watch the progress with
watch cat /proc/mdstat
I am using xfs and my mount point is at /storage
sudo xfs_growfs /storage