ovh server, resize or remove & create partion for boot/logic

This is article for how to make a raid /check them.

This is an official tutorial. 
 This tell us how to grow or shrink a raid partition.
this is how to do it if you are not a raid partition for ovh.

In fact , they all forget one thing, if you are resizing your boot partition, if you have removed the device and recreate it. your UUID is changed, so you need to update the grub.

this article tells us how to reinstall a debian by OVH rescue, and talk about how to correctly update the grub.

The most important thing for update grub after you have successfully resize or remove or recreate your partition is:

mount -obind /dev /mnt/dev/
chroot /mnt/md1
mount none /proc -t proc
apt-get update

grub-install /dev/sda

These action depend on your boot partition is db1, and your db1 array is sda1, sdb1.
do not works , you can try :
    dpkg-reconfigure grub-pc
The most important thing is use
   chroot /mnt/md1
cause you are in a seprated envirement, and you need to modify the md1 system, not the rescue grub. And, you must do :

mount -obind /dev /mnt/dev/

to transfer the modified dev to the new system and for the generation of grub.