Added a Seagate hybrid drive to my mid-2011 Mini.
2 - 4 horas
I bought the mini second hand and it only had a 60G SSD boot drive. An external firewire-800 1T drive does work, but I wanted some SATA speed storage and Newegg.com had a great deal on the 500G recently.
Pretty much flawlessly. Like many others, I would have liked to see the rebuild instructions - but it was not overly hard with a little imagination. Surprisingly the cowling for the fan was probably the hardest thing to put back but mostly because my memory is not what it used to be.
Keep the screws you take out in levels matching the part you are removing so that you know which ones to use at each level on the rebuild.