The second HDD of my Mac mini suffered from physical issues and needed replacement.
I ordered a 1Tb HDD and the HomeTech Toolkit and everything went smooth. I used to DIY on PCs but got a little bit scarred before getting my hands dirty on the MacMini ... but the EXCELLENT repair guide from iFixit made me confident in completing the task.
And voilà! 1 hour latter I was pretty proud of myself when my Macmini was successfully booting and recognizing my HDDs.
The secondary HDD (Macintosh HD2) is the one on the top (first one available) : no need to completely dismantle the board.