title: MongoDB copyDatabase() failure
body: <p>I'm trying to make a copy of a fairly large Mongo database.(about 410GB) using the mongo command db.copyDatabase(). I log in as Admin and issue the command</p>
  
  <pre><code> result = db.copyDatabase("warehouse","tswcopy2")
  </code></pre>
  
  <p>The copy starts and runs for around an hour at which point it terminates with</p>
  
  <p><code>error code 121 "Document failed validation"</code>. </p>
  
  <p>Does anyone have insight? Is it possible that document validation on one of my collections is causing this? </p>