debian - New mdadm RAID vanish after reboot

I have problems with mdadm after reboot, I can't reassemble /dev/md0 .

I work on debian wheezy.

I have done the following steps::

sudo mdadm --create --verbose /dev/md0 --level=6 --raid-devices=4 /dev/sd[b-e]
cat /proc/mdstat
sudo mdadm --readwrite /dev/md0
sudo mdadm --detail --scan >> /etc/mdadm/mdadm.conf
echo check > /sys/block/md0/md/sync_action
sudo pvcreate /dev/md0
sudo pvdisplay
sudo vgcreate vgraid6 /dev/md0
sudo lvcreate -l 100%FREE -n lvHD vgraid6
sudo mkfs.ext4 -v /dev/vgraid6/lvHD

Here all works successfully.

After mounting the RAID, I could use it, create files, access it from other PCs...

Now comes the problem:

After rebooting the server (reboot now), the RAID does not exist anymore, /dev/md0 is gone.

First I checked /etc/mdadm/mdadm.conf:

# mdadm.conf
#
# Please refer to mdadm.conf(5) for information about this file.
#
#DEVICE partitions containers
...
CREATE owner=root group=disk mode=0660 auto=yes
MAILADDR root
ARRAY /dev/md0 metadata=1.2 name=media:0 UUID=cb127a0b:ad4eb61d:e0ba8f82:db4b062d

After I try :

$ mdadm --stop --scan
$ mdadm --assemble --scan

or:

$ sudo  mdadm --assemble /dev/md0 /dev/sd[b-e]
    mdadm: Cannot assemble mbr metadata on /dev/sdb
    mdadm: /dev/sdb has no superblock - assembly aborted


$ sudo  mdadm --examine /dev/sd[b-e]
/dev/sdb:
   MBR Magic : aa55
Partition[0] :   4294967295 sectors at            1 (type ee)
/dev/sdc:
   MBR Magic : aa55
Partition[0] :   4294967295 sectors at            1 (type ee)
/dev/sdd:
   MBR Magic : aa55
Partition[0] :   4294967295 sectors at            1 (type ee)
/dev/sde:
   MBR Magic : aa55
Partition[0] :   4294967295 sectors at            1 (type ee)

The mdadm daemon is running (ps aux | grep mdadm)

empty /proc/mdstat

$ cat /proc/mdstat
Personalities :
unused devices: 

What's wrong?