GRUB unknow file system

i tried to make a Full Disk Encryption to my host OS, it’s Debian Jessie first release .
I made this : encrypt OS partition ( / ) on my external USB HDD - add /boot on external pendrive - GRUB bootloader on pendrive MBR .

After this, i encountered this message : GRUB unknow file system → rescue mode .
Therefore, i don’t know to setup GRUB manually, i found this guide to make a FDE : http://yuxv6qujajqvmypv.onion/ .
I’m sure to do the same things as the guide but it doesn’t work for me .
I know it isn’t a Whonix problem but i think i can ask here :slight_smile: .