Recently I bought an old PC to start using as a home server for storage, running a game server, etc. I bought a PC that was listed as having a 1 TB HDD, and I was expecting to just buy a 128 GB SATA SSD as a boot drive for my OS and any other critical things on....

NixOS is a Linux distribution that is completely and entirely reproducible. Everything you use is defined in a configuration file that is used to build your system. All the services, packages, options, partition layout, hardware, everything, is in this config file....