Got this fine hint from the gentoo grub error collection
Could Not Find Device For /boot/boot: Not Found Or Not a Block Device
Situation
When running grub-install during the GRUB installation, you receive the
following error:
Grub Output
# grub-install –root-directory=/boot /dev/hda
Could not find device for /boot/boot: not found or not a block device
Solution
Check that you didn’t forget the following:
Updating the mtab file
# cp /proc/mounts /etc/mtab
213/433