--- update-grub	2008-08-31 20:36:24.000000000 +0200
+++ update-grub-new	2008-12-18 17:36:14.000000000 +0100
@@ -215,7 +215,11 @@
 
 # Drive(in GRUB terms) where the kernel is located. Overridden by the
 # kopt statement in menufile.
-grub_root_device=$(convert_default "$boot_device")
+if ! grub_root_device=$(convert_default "$boot_device"); then
+    echo -e "Grub root device not found. Is $device_map is outdated?"
+    echo -e "Run grub --device-map $device_map and try again"
+    exit -1
+fi
 
 # should grub create the alternative boot options in the menu
 	alternative="true"