If you get the error
bad hypermeta data when trying to mount an ext3
partition from GNU/Linux, that is usually because the file system has not been
unmounted cleanly (maybe GNU/Linux got suspended to disk) and the Hurd cannot
mount it as ext2 without checking. Either boot back into GNU/Linux and unmount
it or you can try to run
fsck.ext3 from GNU/Hurd directly.