Here is a possible patch for:
util/grub.d/30_os-prober.in
to provide the correct device name for booting the Hurd (it expects device names like:
hd0s2
instead of:
(hostdisk//dev/sda,msdos2)
provided currently).
The patch was originally submitted as a Debian bug at:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=825292

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diff -Naur grub-git/util/grub.d/30_os-prober.in grub-git-2/util/grub.d/30_os-prober.in
--- grub-git/util/grub.d/30_os-prober.in        2016-05-31 20:00:59.770965288 +0200
+++ grub-git-2/util/grub.d/30_os-prober.in      2016-05-31 20:03:04.526960041 +0200
@@ -298,7 +298,7 @@
       save_default_entry | grub_add_tab
       prepare_grub_to_access_device ${DEVICE} | grub_add_tab
       grub_device="`${grub_probe} --device ${DEVICE} --target=drive`"
-      mach_device="`echo "${grub_device}" | sed -e 's/(\(hd.*\),msdos\(.*\))/\1s\2/'`"
+      mach_device=`${grub_probe} --device ${DEVICE} --target=compatibility_hint | sed 's/,msdos/s/'`
       grub_fs="`${grub_probe} --device ${DEVICE} --target=fs`"
       case "${grub_fs}" in
        *fs)    hurd_fs="${grub_fs}" ;;
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