moving files from Solid State Drive to secondary Hard Disk Drive

I have a Mac Mini (Late 2012). I recently upgraded my storage from my original 1 TB hard drive, to an additional 500 GB solid state drive. My operating system is on the new solid state drive. What I would like to do is move as many files,etc. that I can from the SSD to the HDD, so that I can free up space on the SSD to install Windows 10 via Bootcamp.
As shown in the attached screenshot, almost 350 GB of my total SSD storage is taken up by "Other".

My question is, how do I find out what exactly is included in this "other" category, and are there any restrictions on moving some, or all, of these files to my secondary hard drive?

"Other" is a catch all term for any files on the drive that do not fit into one of the other categories. There are some utilities out there that can show you what the files are and how much space is being used. Two of the favorites seem to be DaisyDisk and GrandPerspective.

You can easily move the iPhoto/Photos library to an external drive was well as the iTunes library if you don't mind needing to have the drive attached when running those programs. These are the most common culprits for most users. When performing this kind of task I tend to suggest the following rules.:
1. Mack sure to perform a full backup (preferably a clone) before deleting anything. If you accidentally delete something at least you have a path to restore things.
2. If you don't recognize a file as something you created leave it where it is until you know for sure what it is/does.

Slydude,

Thanks so much for the prompt response. I tried the free trial of DaisyDisk, and it told me exactly what I needed to know.

You're welcome. Glad that helped.