Asus x550 SSD+HDD upgrade

This is not a full tutorial!! If you try this and you don’t know what you are doing don’t blame me for screwing your device!! This type of upgrade can be made on any laptop that has an optical drive. What size for the 2nd HDD caddy do I need for my laptop??: Laptop configuration: Asus X550LB-XX021D Intel® CoreTM i5-4200U , Haswell nVidia GeForce G740 2GB (GK208, 64bit) 4GB Ram On Board 120gb SSD Kingston 750GB HDD Samsung Second hdd caddy Song: