Running Ubuntu Lucid/Mac OS X Snow Leopard on a a Mac Pro.

My problem is that GRUB won't boot properly. I choose "boot linux from HD" at the rEFIt menu, but then I get stuck at something like this
Minimal BASH editing is supported. TAB generates a list of possible commands etc.

grub>
Ubuntu works pretty much fine when I boot it from super grub disk, so I am (almost) sure GRUB is the problem.

Appreciate any help

Daniel