2 - 4 horas
The mac mini has room for dual 2.5" drives but doesn't include the cabling for a second drive.
Kinda a pain but it worked out ok. Outrageously expensive for a simple cable but this isn't really iFixit's fault, I'm sure apple's wholesale price for the cable alone is astronomical.
When sliding the logic board out of the mac mini case, note that the black plastic IO plate has snap-in tabs on either side - it doesn't just slide out effortlessly. Before I realized this detail, I was struggling with sliding the board, it kept wanting to slide unevenly and I was afraid I'd break something. Once I was sure I hadn't missed a screw somewhere I applied a little extra force and it snapped loose without damaging the board.