title: MongoDB addshard: failed to add 2nd shard
body: <p>There are 6 directories, each contains about 1 TB data, they all belong to a database "test"
  now, and I want group them in a cluster. I have built the Shardserver and Configserver,  but encounter a problem when adding the 2nd shard to the cluster:</p>
  
  <pre><code>db.runCommand({addshard:localhost:27017});
  db.runCommand({addshard:localhost:27027});
  </code></pre>
  
  <blockquote>
    <p>"errmsg" : "can't add shard
    localhost:27027 because a local database 'test' exists in another
    shard0000:localhost:27017"</p>
  </blockquote>
  
  <p>Dropping the 'test' database can solve this problem, but it is not what I want.</p>
  
  <p>How can I merge the 6 shards (they all have 'test' database, belonging to a single logical database) together?</p>