diff --git a/hw/usb-msd.c b/hw/usb-msd.c
index 01d492d42d..f7ad25e62e 100644
--- a/hw/usb-msd.c
+++ b/hw/usb-msd.c
@@ -517,13 +517,42 @@ USBDevice *usb_msd_init(const char *filename)
 {
     MSDState *s;
     BlockDriverState *bdrv;
+    BlockDriver *drv = NULL;
+    const char *p1;
+    char fmt[32];
+
+    p1 = strchr(filename, ':');
+    if (p1++) {
+        const char *p2;
+
+        if (strstart(filename, "format=", &p2)) {
+            int len = MIN(p1 - p2, sizeof(fmt));
+            pstrcpy(fmt, len, p2);
+
+            drv = bdrv_find_format(fmt);
+            if (!drv) {
+                printf("invalid format %s\n", fmt);
+                return NULL;
+            }
+        } else if (*filename != ':') {
+            printf("unrecognized USB mass-storage option %s\n", filename);
+            return NULL;
+        }
+
+        filename = p1;
+    }
+
+    if (!*filename) {
+        printf("block device specification needed\n");
+        return NULL;
+    }
 
     s = qemu_mallocz(sizeof(MSDState));
     if (!s)
         return NULL;
 
     bdrv = bdrv_new("usb");
-    if (bdrv_open(bdrv, filename, 0) < 0)
+    if (bdrv_open2(bdrv, filename, 0, drv) < 0)
         goto fail;
     if (qemu_key_check(bdrv, filename))
         goto fail;
diff --git a/qemu-doc.texi b/qemu-doc.texi
index 636d819281..9519d9a072 100644
--- a/qemu-doc.texi
+++ b/qemu-doc.texi
@@ -550,8 +550,10 @@ Pointer device that uses absolute coordinates (like a touchscreen). This
 means qemu is able to report the mouse position without having to grab the
 mouse. Also overrides the PS/2 mouse emulation when activated.
 
-@item disk:file
-Mass storage device based on file
+@item disk:[format=@var{format}]:file
+Mass storage device based on file. The optional @var{format} argument
+will be used rather than detecting the format. Can be used to specifiy
+format=raw to avoid interpreting an untrusted format header.
 
 @item host:bus.addr
 Pass through the host device identified by bus.addr (Linux only).