Error creating Raid 5

Hi i'm trying to create a Raid 5 in two phases but i get this error
Can anyone help me?
- Code: Select all
root@fvdwsl-base:/ # umount /dev/sdb8
root@fvdwsl-base:/ # umount /dev/sdc8
root@fvdwsl-base:/ # umount /dev/sdd8
root@fvdwsl-base:/ # umount /dev/sde8
root@fvdwsl-base:/ # mdadm --create /dev/md0 --level=5 --raid-devices=5 missing /dev/sdb8 /dev/sdc8 /dev/sdd8 /dev/sde8
mdadm: error opening /dev/md0: No such device or address
Can anyone help me?