This might prove useful.
Teardown specialist and accessory manufacturer iFixit has announced that the company has released its Mac Mini Dual Hard Drive Kit. While the kit doesn’t come with an additional hard drive, it provides you with the tools to install one, including a proprietary SATA hard drive cable, four shock-absorbing mounting grommets, four mounting screws, a Mac Mini Logic Board Removal Tool, a 26-Piece Bit Driver Kit, and a handy spudger.
Users can install either a solid-state or traditional spinning hard drive, depending on your preferences.
Though iFixit lists this repair as “difficult,” the company does provide a 38 installation guide along with the product.
Users can purchase the Mac Mini Dual Hard Drive Kit from iFixit’s website for US$70 and you’ll need a 2011-era 2.33GHz, 2.5GHz, or 2.7GHz Mac Mini (Model A1347).