Index: /trunk/src/VBox/Storage/VD.cpp
===================================================================
--- /trunk/src/VBox/Storage/VD.cpp	(revision 49879)
+++ /trunk/src/VBox/Storage/VD.cpp	(revision 49880)
@@ -2421,5 +2421,6 @@
     pIoCtx->fFlags |= VDIOCTX_FLAGS_ZERO_FREE_BLOCKS;
 
-    if (pIoCtx->Req.Io.cbTransferLeft)
+    if (   pIoCtx->Req.Io.cbTransferLeft
+        && !pIoCtx->cDataTransfersPending)
         rc = vdReadHelperAsync(pIoCtx);