Because of cache coherency problem grub accesses partitions on linux by
hdaX device and not by hda with correct offset. The problem is that
because of 4K cache blocks disk.c may read sectors before the partition
and hence making hostdisk.c try to read from negative offset. I'm sad
that we need such workarounds for free systems.

-- 
Regards
Vladimir 'φ-coder/phcoder' Serbinenko