Código del archivo file.php de las líneas 250 a la 283:
/**
 * Get absolute file for a given key
 *
 * @param string $key The key
 * @return mixed Absolute cache file for the given key or false if erroneous
 * @access private
 */
   function _setKey($key) {
      $this->_File->Folder->cd($this->settings['path']);
      if ($key !== $this->_File->name) {
         $this->_File->name = $key;
         $this->_File->path = null;
      }
      if (!$this->_File->Folder->inPath($this->_File->pwd(), true)) {
         return false;
      }
   }
/**
 * Determine is cache directory is writable
 *
 * @return boolean
 * @access private
 */
   function __active() {
      if ($this->_init && !is_writable($this->settings['path'])) {
         $this->_init = false;
         trigger_error(sprintf(__('%s is not writable', true), $this->settings['path']), E_USER_WARNING);
         return false;
      }
      return true;
   }
}