Some models can be very fussy to setup if it doesn't a capacity to have multiple HDD/SSD as a stock option, this includes switching out an optical drive with a HDD or SSD. It also doesn't always play nice with an external optical drive if you switch one out (e.g., installing Windows via BootCamp may not work since BootCamp won't recognize an external drive). Oh, if you need to do a clean install of OS, it can potentially mean taking out the Data Doubler. YMMV, of course, depending on your Mac.