--- grub-pc.postinst.org	2018-09-07 13:52:19.609236137 +1200
+++ grub-pc.postinst.new	2018-09-07 13:52:58.000000000 +1200
@@ -528,7 +528,8 @@
             if [ -z "$new_devices" ]; then
               new_devices="$(devices_to_ids $old_devices)"
               db_set grub-pc/install_devices "$new_devices"
-              # Common-case optimisation: if all devices are translatable
+              # Common-case optimisations: if the devices where already
+	      # in the new form or if all devices are translatable
               # to by-id and the number of devices there is the same as
               # the number of devices GRUB can see, then there's no point
               # asking again.  (This handles e.g. "/dev/sda" with one
@@ -536,8 +537,9 @@
               old_devices_count="$(echo "$old_devices" | wc -w)"
               new_devices_count="$(echo "$new_devices" | wc -w)"
               devices_count="$(echo "$devices" | wc -w)"
-              if [ "$old_devices_count" != "$new_devices_count" ] || \
-                 [ "$new_devices_count" != "$devices_count" ]; then
+              if [ "x$old_devices" != "x$new_devices" ] &&
+		 { [ "$old_devices_count" != "$new_devices_count" ] || \
+                   [ "$new_devices_count" != "$devices_count" ]; }; then
                 db_fset grub-pc/install_devices seen false
                 db_fset grub-pc/install_devices_empty seen false
               fi