This 2nd HDD Caddy(Hard Drive Adapter) for Asus x51 lets you add a second hard drive to the laptop, it is swap the original CD,DVD or Blu-ray Optical drive.
If you are not sure this will work for your notebook, please email the dvd model number, which you could see in the control panel-->system-->hardward-->device manger-->dvd rom, you should be able to see the model number listed. We will help you know which one to use and buy.