Ever since it became clear that the 2011 Mac minis could be easily upgraded to swallow two HDDs I wanted to do the modifications.
For now this would allow me to have two HDDs inside the mini, later on, I'd swap one for a small but fast SSD.
Buy iFixit's Mac mini HDD upgrade kit and get a second HDD.
I decided to buy a 500 GB Western Digital Scorpio Black, which is the fastest 9.5 mm HDD out there, as far as I can tell. It serves as my boot drive now.
The repair went well except for one problem: The screw that holds the fan to the cowling (Step 3, left screw) must've had too much LockTite applied, because while unscrewing it, the screw below came loose.
Nothing would make these two screws detach, resulting in a difficult reassembly, where I had to put in the cowling and the fan simultaneously, taking care to add a small spring into the hole below the cowling.
Other than that everything went beautifully. The Mac mini works perfectly and the secondary HDD works as my Time Machine drive, for speedy backups of my personal files.
Get the recommended nylon spudger, it makes prying thins off way easier and safer, get some decent tweezers from your local hardware store.
For further peace of mind, I recommend removing the RAM from the logic board immediately after opening the Mac mini.
Also: Don't panic.