I finally managed to recover all the space instead of keeping only raid Debian Jessie (WD mycloud gen1).
I used GParted from a live cd, I did not succeed in ssh. So it’s simple:
[Code]small forgotten before handling in mycloud ssh :
#remove partition 2 from raid:
mdadm /dev/md1 --fail /dev/sda2
mdadm /dev/md1 --remove /dev/sda2
With LiveCD/USB only :
Delete partiton /dev/sdb2/ with Gparted
Resize /dev/sdb1/ 100% always with gparted
#Back on mycloud ssh :
mdadm --grow /dev/md1 --size max
By cons, obligation to format /dev/sdb4/ to ext4 (lost data), otherwise the drive will be read-only. [/Code]
Nothing complicated, it rose to 3.8 GB for OMV or Debian, it helps out well 
