def get_free_mb(folder):
    """ Return folder/drive free space (in bytes)
    """
    if platform.system() == 'Windows':
        free_bytes = ctypes.c_ulonglong(0)
        ctypes.windll.kernel32.GetDiskFreeSpaceExW(ctypes.c_wchar_p(folder), None, None, ctypes.pointer(free_bytes))
        return free_bytes.value/1024/1024
    else:
        st = os.statvfs(folder)
        return st.f_bavail * st.f_frsize/1024/1024