Index: /trunk/src/VBox/Main/MediumImpl.cpp
===================================================================
--- /trunk/src/VBox/Main/MediumImpl.cpp	(revision 32933)
+++ /trunk/src/VBox/Main/MediumImpl.cpp	(revision 32934)
@@ -3860,5 +3860,11 @@
                 ComAssertRCThrow(vrc, E_FAIL);
 
-                if (isImport)
+                /* streamOptimized VMDK images are only accepted as base
+                 * images, as this allows automatic repair of OVF appliances.
+                 * Since such images don't support random writes they will not
+                 * be created for diff images. Only an overly smart user might
+                 * manually create this case. Too bad for him. */
+                if (   isImport
+                    && !(uImageFlags & VD_VMDK_IMAGE_FLAGS_STREAM_OPTIMIZED))
                 {
                     /* the parent must be known to us. Note that we freely