diff --git a/lib/config.py b/lib/config.py
index 1d792d1c5c0f7edab77f9fc8fa56d7b73ba269f3..9256bb958f7ca22e9e0175b705cc9acf49fc6a35 100644
--- a/lib/config.py
+++ b/lib/config.py
@@ -269,6 +269,11 @@ class ConfigWriter:
           ports[net_port] = []
         ports[net_port].append((instance.name, "network port"))
 
+      # instance disk verify
+      for idx, disk in enumerate(instance.disks):
+        result.extend(["instance '%s' disk %d error: %s" %
+                       (instance.name, idx, msg) for msg in disk.Verify()])
+
     # cluster-wide pool of free ports
     for free_port in data.cluster.tcpudp_port_pool:
       if free_port not in ports:
diff --git a/lib/objects.py b/lib/objects.py
index 89473fec7cc1e8e904f962334f219d27a3443668..0feefbedfc2a7114bda715f7562fc86fe9ceac69 100644
--- a/lib/objects.py
+++ b/lib/objects.py
@@ -497,6 +497,15 @@ class Disk(ConfigObject):
     val += ", size=%dm)>" % self.size
     return val
 
+  def Verify(self):
+    """Checks that this disk is correctly configured.
+
+    """
+    errors = []
+    if self.mode not in constants.DISK_ACCESS_SET:
+      errors.append("Disk access mode '%s' is invalid" % (self.mode, ))
+    return errors
+
 
 class Instance(TaggableObject):
   """Config object representing an instance."""