title: Docker volume on external hard drive
body: <p>I'm using a postgres image to run a container locally. 
  I need to process a significant amount of data and I want to use an external hard drive for this. Is it possible to mount a volume on an external hard drive? Can I accomplish what I want with <a href="https://github.com/ClusterHQ/flocker" rel="noreferrer">Flocker</a>?</p>
  
  <p>I'm using native docker for mac.</p>