The new Mac mini has plenty of space inside for a second hard drive, but Apple charges a huge premium to install a second drive or SSD inside. There’s no reason why you shouldn’t be able to take advantage of the space. To help you out, the folks over at iFixit have come up with Mac Mini Dual Hard Drive Kit ($70), which has all the pieces you need, except for the hard drive, which you choose yourself.
Inside the Dual Hard Drive Kit are a proprietary SATA hard drive cable, shock-absorbing mounting grommets, mounting screws, a Mac mini Logic Board removal tool, a 26-piece bit drive kit, and a spudger to open the computer. You can plug in any standard 2.5” SATA drive, but it’s easiest to recommend adding an SSD, which can really speed the mini up if used as a boot drive. Fair warning: the company lists this as a difficult installation, and any time you start poking around inside your Mac, it voids the warranty. You can decide whether this is a small price to pay to turn your mini into a more powerful little monster.