s.RUnlock()
		return diskID, nil
	}
	s.RUnlock()

	fi, err := s.checkFormatJSON()
	if err != nil {
		return "", err
	}

	if xioutil.SameFile(fi, fileInfo) && diskID != "" {
		s.Lock()
		// If the file has not changed, just return the cached diskID information.
		s.formatLastCheck = time.Now()
		s.Unlock()
		return diskID, nil
	}