code: func (u *storageosUtil) UnmountVolume(b *storageosUnmounter) error {
  	if err := u.NewAPI(b.apiCfg); err != nil {
  		// We can't always get the config we need, so allow the unmount to
  		// succeed even if we can't remove the mount reference from the API.
  		klog.V(4).Infof("Could not remove mount reference in the StorageOS API as no credentials available to the unmount operation")
  		return nil
  	}
  
  	opts := storageostypes.VolumeUnmountOptions{
  		Name:      b.volName,
  		Namespace: b.volNamespace,
  		Client:    b.plugin.host.GetHostName(),
  	}
  	return u.api.VolumeUnmount(opts)
  }
docs: // Unmount removes the mount reference from the volume allowing it to be
  // re-mounted elsewhere.