public function realpath($file)
    {
        $prefix = preg_quote($this->_prefix, '~');
        $file = preg_replace("~^{$prefix}~", '', $file);
        if (!$this->_hasVolume) {
            return $file;
        }
        if (preg_match('~^[A-Z]+:~', $file)) {
            return $file;
        }
        $file = ltrim($file, DS);
        $pos = strpos($file, DS);
        if ($pos !== false) {
            $file = substr_replace($file, ':' . DS, $pos, 1);
        }
        return $file;
    }