You used some very nice hardware. I was going to recommend you upgrade those hard drives because the 250 and 500 GB will be so slow compared to a modern 7200 rpm drive. But you already got some new but slow 5400 rpm drives .
In my Gaming pc use the following storage
1: 250 GB SSD for OS, program files and personal data
2: 2x 1 TB Seagate Desktop HDD in raid0 (reaches 400 MB/s+ sequentially, very useful while installing) for games
3: 1x 3 TB Seagate Desktop HDD for video recording and big data I need locally
All other drives I put in a self build NAS and I have not regretted it since. I can really recommend moving stuff like your gaming backups to another machine. I literally did just that: take out the drive and move it to a NAS. A basic Gbit connection is good for 130 MB/s so with those 5400 rpm WD Green drives you will hardly notice they are not local. You can even mount them and use them like a normal drive if you wish.
edit: Can't reply anymore so I will do an edit.
The M2 SSD is awesome. But the PC still has these drives right?
3TB Western Digital Caviar Green Sata2 HD [G + D] [hotswap] $ 128
2TB Western Digital Caviar Green Sata2 HD [I] [hotswap] $ 99
2TB Western Digital Caviar Green Sata2 HD [J] [hotswap] $ 99
I recommended moving those to a separate NAS . I hated it when I went to "computer" and they all started spinning up. I went from 6 disks to 3. The Raid 0 I use to fit my 980 GB of installed games. Let's hope 1 TB NVME SSD's become affordable soon ^^.