Index: /trunk/src/VBox/Storage/VDI.cpp
===================================================================
--- /trunk/src/VBox/Storage/VDI.cpp	(revision 37551)
+++ /trunk/src/VBox/Storage/VDI.cpp	(revision 37552)
@@ -1107,5 +1107,5 @@
         pvUser = pIfProgress->pvUser;
     }
-    
+
     /* Check the image flags. */
     if ((uImageFlags & ~VD_VDI_IMAGE_FLAGS_MASK) != 0)
@@ -1355,5 +1355,6 @@
             }
 
-            if (cbToWrite == getImageBlockSize(&pImage->Header))
+            if (   cbToWrite == getImageBlockSize(&pImage->Header)
+                && !(fWrite & VD_WRITE_NO_ALLOC))
             {
                 /* Full block write to previously unallocated block.