Great Instructions, gets you through with ease!
I bought a used Mac Mini, that only had 2 GB RAM, and had 10.7.5 installed on an OEM 5400 rpm standard drive. I wanted my wife to have a really nice computing experience, so I thought I'd upgrade to double HDs, with the new one being an SSD.
I threw in a 2nd drive--a 240 GB SSD, and installed 10.11.6 on that to run the Mini. Added 8 GB of RAM to top it off. The iFixit instructions were so clear, even my wife commented on how good they were. I was in and out in under an hour, and my only problem was the top drive wouldn't line up properly, which made it impossible to re-install all four screws in the antenna plate. I had to leave one out. Mini works beautifully, quite an upgrade from her old 2008 iMac 1.83 GHz 2 GB . Thank you iFixit for the great instructions, made the job go smoothly.
If you're going to add a 2nd HD to a Mac Mini, be sure to test fit the drives to make sure the ribbon connector sit as flat as possible to make room for the top drive to sit down flush..