At the back of the Core you will see a removable caddy tray: you can use a small cross-headed screwdriver to remove the two screws on the caddy. Slide the caddy out and place your chosen HDD on the caddy, carefully turn it over and insert the provided HDD/SSD screws into the marked holes for your HDD/SSD to secure it in place making sure the SATA connection for the drive is mounted towards the internal connections of the Core. Slide the caddy back into the Core and replace the screws on the caddy tray.