Index: /trunk/src/VBox/Storage/VD.cpp
===================================================================
--- /trunk/src/VBox/Storage/VD.cpp	(revision 50851)
+++ /trunk/src/VBox/Storage/VD.cpp	(revision 50852)
@@ -1912,5 +1912,5 @@
     RTSgBufInit(&SgBuf, &Segment, 1);
     vdIoCtxInit(&IoCtx, pParentState->pDisk, VDIOCTXTXDIR_READ, uOffset, cbRead, pParentState->pImage,
-                &SgBuf, NULL, NULL, VDIOCTX_FLAGS_SYNC);
+                &SgBuf, NULL, NULL, VDIOCTX_FLAGS_SYNC | VDIOCTX_FLAGS_ZERO_FREE_BLOCKS);
     int rc = vdReadHelperAsync(&IoCtx);
     ASMAtomicXchgBool(&pParentState->pDisk->fLocked, false);