def _other_versions_on_disk():
            """Returns previous versions on disk."""
            if not os.path.exists(builder_data_dir):
                return []

            version_dirnames = []
            for dir_name in os.listdir(builder_data_dir):
                try:
                    version_dirnames.append((utils.Version(dir_name), dir_name))
                except ValueError:  # Invalid version (ex: incomplete data dir)
                    pass
            version_dirnames.sort(reverse=True)
            return version_dirnames