Index: head/sys/kern/subr_bus.c
===================================================================
--- head/sys/kern/subr_bus.c
+++ head/sys/kern/subr_bus.c
@@ -3004,6 +3004,10 @@
 	PDEBUG(("%s", DEVICENAME(dev)));
 	if (dev->state == DS_BUSY)
 		return (EBUSY);
+	if (dev->state == DS_ATTACHING) {
+		device_printf(dev, "device in attaching state! Deferring detach.\n");
+		return (EBUSY);
+	}
 	if (dev->state != DS_ATTACHED)
 		return (0);