Mac mini's internal HDD was nearly full. But there it's enough space for another hard disk drive or even a SSD. Don't buy apple's Mac mini server, build your own one!
First you have to order a suitable HDD drive cable. You need some tools to open Mac mini's body an pull out his heart and guts.
I bought ifixit's Mac Mini Dual Drive Kit with all necessary tools & parts & manuals to do this job and it helped me a lot!
After opening the case and removing the logic board, I was able to install another 1TB HDD drive. Putting all things back together the Mac mini will identify the second drive and is up and running again.
Use the right tools. Never try to disassemble the mini without proper screwdrivers, or you will get screwed. Always trust ifixit as I did and the world will be a better place.
Thank you, ifixit!