hetzner resque help to install grub

Hi ALL,
hetzner resque: help to install grub after raid 1 synchronization
last help request before re-format ALL machine...

my souce hd was sdb

I successful make ALL in this guide:
http://wiki.hetzner.de/index.php/Festplattenaustausch_im_Software-RAID/en#Preparing_the_new_drive

BUT when I try to:
grub-install /dev/sdb

I receive:
/usr/sbin/grub-probe: error: cannot find a device for /boot/grub (is /dev mounted?).

so I've just try:
mount /dev/md0 /mnt

I receive:
mount: /dev/md0 already mounted or /mnt busy
mount: according to mtab, /dev/md0 is already mounted on /mnt

Suggestions ?
thx
GioMBG