After nearly two months of not being able to boot up my Mac mini, I tried last night and it worked. Yay!
First things first. I moved my photos library from my Mac mini to external storage. I am currently deleting a number of libraries from my Mac mini.
I take a lot of photos. My current library is 218 GB. No wonder my Mac mini fails to boot up, especially with all the library copies that I have. There simply isn't enough memory to do anything.
Here's to freeing up hard disk space.