If you've already installed the drive, just select it as the start up device in system preferences. If you have not installed it, use this: Mac Mini Dual Drive Kit
Add a second hard drive to your 2011 or 2012 Mac Mini. Kit includes: SATA Hard Drive Cable, Mounting Grommets, Mounting Screws, Logic Board Removal Tool, 26 Bit Driver Kit, and ESD-Safe Spudger. Compatible with 2.3, 2.5, or 2.7 GHz 2011 Mac Minis (Model A — 50+ available at 29.95 each.