code: func (d *Driver) PreCreateCheck() error {
  	// Downloading boot2docker to cache should be done here to make sure
  	// that a download failure will not leave a machine half created.
  	b2dutils := mcnutils.NewB2dUtils(d.StorePath)
  
  	return b2dutils.UpdateISOCache(d.Boot2DockerURL)
  }
docs: // PreCreateCheck checks that the machine creation process can be started safely.