The normal boot option is HDD first. So this is the main reason you should place the ssd with the os of course in the hdd's place. In disassembling the unit you will have to open the backcase. See hardware maintenance manual. The fsteps in sequence are the screws holding the keyboard, the keyboard, the ODD, the backcase screws, and lastly the backcase to access the hdd. If you place the ssd in the caddy and decide to boot on it you will be accessing the boot menu everytime you restart the computer. If you decide to just place it in the caddy without booting from it, it defeats the purpose of the ssd