def _unsafe_split_mgm(filename):
    """
    Takes a properly formatted path starting with 'root:' and containing '/store'
    """
    if not filename.startswith('root://'):
        raise ValueError('Cannot split mgm; passed filename: {0}'.format(
            filename))
    elif not '/store' in filename:
        raise ValueError("No substring '/store' in filename {0}".format(
            filename))
    i = filename.index('/store')
    mgm = filename[:i]
    lfn = filename[i:]
    return mgm, lfn
