Question regarding --root-device and --prefix parameters

[Πρεκατές Αλέξανδρο ς]

Hi from greece.

I'm troubled about the semantics of  the --root-device parameter in 
grup-setup and 
--prefix in grub-mkimage. 
For time being  i have the impression that they both are related with
what files/dirs core.img will have access to, so the latter could 
override the first.

thanks.

ps:The context of my question is trying to set up a dedicated grub2 
partition , and
having troubles making installed grub (i used grub-install 
--root-directory=/mnt/grub2 /dev/hda )
find the grub.cfg i had in grub2 partition /dev/hda2.