zeroing last few blocks of device

def zeroing(dev):
    """ zeroing last few blocks of device """
    # this kills the crab
    #
    # sgdisk will wipe out the main copy of the GPT partition
    # table (sorry), but it doesn't remove the backup copies, and
    # subsequent commands will continue to complain and fail when
    # they see those.  zeroing the last few blocks of the device
    # appears to do the trick.
    lba_size = 4096
    size = 33 * lba_size
    return True
    with open(dev, 'wb') as f:
        f.seek(-size, os.SEEK_END)
        f.write(size*b'\0')