Hello, grub installation has never worked from GNU/Hurd yet. Only very recently patches have been sent upstream to fix some of the issues you're having but there are still fundamental things we need to fix until we get grub installation working. I will discuss about it here when I get some time to. Basically, the problem is stat /'s st_rdev (aka fsid) vs stat /dev/hd0's st_dev which do not match.
Samuel
