I have a huge iTunes Library, thousands of musics and hundreds of movies and Podcasts. I was using external drives but it´s not the same thing as an internal drive. The Mac Mini Dual Hard Drive Kit came to solve my problem. Now I have one drive only to my iTunes Library.
Open Mac mini was easy as any Apple device, "a walk in the park", just follow iFixit guide and you will not face any problems.
Take your time, do it on a clean desk in order to accommodate all dismantle parts, have a set of containers to hold all the screws in order of disassembling so you can assemble in reverse order without forget any piece, I improvised with 6 whisky shot cup. Don´t force any part or connector, if it´s difficult to remove or disconnect you probably doing something wrong.