public function getDisk()
    {
        if ($this->disk_instance === null) {
            // Create disk class
            $class = "\\Torann\\MediaSort\\Disks\\" . ucfirst($this->config('disk'));

            // Verify disk
            if (class_exists($class) === false) {
                throw new InvalidClassException("Disk type \"{$class}\" not found.");
            }

            // Create disk instance
            $this->disk_instance = Container::getInstance()->makeWith($class, ['media' => $this]);
        }

        return $this->disk_instance;
    }