Well, grml2hd can't guess what devices will be available on next reboot.

So all you can do is adjust groot/root lines in /boot/grub/menu.lst according 
to your needs (and run update-grub afterwards) and take a look at /boot/grub/
device.map.

Sorry, I don't have any idea how I should "fix" that within grml2hd itself, 
please let me know if you know any workarounds.