The 2nd HDD Caddy is a optical drive bay for a hard drive which you can mount inside your laptop. This caddy fits perfectly in the bay where normally your optical drive (dvd-drive) is located. This allows you to swap your optical drive with the HDD Caddy.
If you want to add a second hard drive, then you can order this 2nd hdd hard drive caddy for your laptop. The installation of the HDD Caddy can be done in ten minutes. You can often remove the dvd-drive by taking out a screw at the bottom of your laptop. When the screw is removed you can take out the dvd-drive and replace it with HDD Caddy and a second hard drive. Then put the screw back in.