code: def get_dev_at_mountpoint(mntpoint):
          
          results = util.subp(['findmnt', '-o', 'SOURCE', mntpoint])
          if results.return_code != 0:
              raise MountError('No device mounted at %s' % mntpoint)
  
          stdout = results.stdout.decode(sys.getdefaultencoding())
          return stdout.replace('SOURCE\n', '').strip().split('\n')[-1]
docs: Retrieves the device mounted at mntpoint, or raises
          MountError if none.