def _init_file(self):
        """Initialise the file header. This will erase any data previously in the file."""
        header_length = 2*SECTOR_LENGTH
        if self.size > header_length:
            self.file.truncate(header_length)
        self.file.seek(0)
        self.file.write(header_length*b'\x00')
        self.size = header_length
Initialise the file header. This will erase any data previously in the file.