Sculpt Branch:

Revised external multires file saving. Now it is more manual in that you
have to specify where to save it, like an image file, but still saved at
the same time as the .blend. It would ideally be automatic, but this is
difficult to implement, so for now this should at least be more reliable.

release/scripts/ui/properties_data_modifier.py

@@ -438,7 +438,6 @@ class DATA_PT_modifiers(DataButtonsPanel):
         col.prop(md, "levels", text="Preview")
         col.prop(md, "sculpt_levels", text="Sculpt")
         col.prop(md, "render_levels", text="Render")
-        col.prop(md, "optimal_display")
 
         if wide_ui:
             col = split.column()
@@ -446,9 +445,20 @@ class DATA_PT_modifiers(DataButtonsPanel):
         col.enabled = ob.mode != 'EDIT'
         col.operator("object.multires_subdivide", text="Subdivide")
         col.operator("object.multires_higher_levels_delete", text="Delete Higher")
+        col.prop(md, "optimal_display")
+
+        layout.separator()
+
+        col = layout.column()
         row = col.row()
-        row.enabled = md.total_levels > 0
-        row.prop(md, "external")
+        if md.external:
+            row.operator("object.multires_pack_external", text="Pack External")
+            row.label()
+            row = col.row()
+            row.prop(md, "filename", text="")
+        else:
+            row.operator("object.multires_save_external", text="Save External...")
+            row.label()
 
     def PARTICLE_INSTANCE(self, layout, ob, md, wide_ui):
         layout.prop(md, "object")