The Mac Mini Uses a 2 1/2" Laptop Hard Drive
If you need to replace the hard drive in your Mac Mini, be aware that it takes a 2 1/2" laptop hard drive, or SSD hard drive. Installing a new hard drive requires complete disassembly of the Mac Mini, and should be done by professionals, or competent do it yourselfers. Splitting the Mac Mini apart can be challenging.
In order to replace the hard drive in a Mac Mini, it has to be split apart, and the logicboard removed. This takes skill, and also special tools. Be sure to do all of your homework before you start prying on your Mac Mini's case, or take it to a professional to install the new hard drive, and load the MacOs.