Index: /trunk/src/VBox/Devices/Storage/ISCSIHDDCore.cpp
===================================================================
--- /trunk/src/VBox/Devices/Storage/ISCSIHDDCore.cpp	(revision 29497)
+++ /trunk/src/VBox/Devices/Storage/ISCSIHDDCore.cpp	(revision 29498)
@@ -2315,4 +2315,7 @@
     AssertRC(rc);
     fHostIPDef = !!uHostIPTmp;
+
+    if (uOpenFlags & VD_OPEN_FLAGS_ASYNC_IO)
+        return VERR_NOT_SUPPORTED;
 
     pImage->uOpenFlags      = uOpenFlags;