I require more space: Suggestions?

Greetings.

Recently, I have constructed my first computer with a 1tb NVME for gaming. I have learned that, as I have not done any modern gaming for the last ten years or so on a computer, that files sizes have bloated so severely that most of my drive has been consumed by just a handful of games. I need more space. My board seemingly has a slot available for another NVME card, and has a bay beneath for a SSD, but I have the 4tb Seagate Barracuda from my last computer that unfortunately the mounting brackets will not actually accept.

1. Is it safe to just leave the Seagate on the bottom of my computer case and run its SATA cable up to the motherboard to connect it? Should I find an actual mounting bracket for it or make do with what I have?
2. Would an external hard drive be better for this, and what would you recommend?

Besides games, I store many other things. Movies, photos, videos I captured, and I do not wish to lose them. I have photos and videos going back to when I was a boy in the early 2000s, and would like to make copies so that they cannot be lost. Movies and interesting things, like the incomplete Battletech game for the Sega CD, are also something I wish to keep ahold of. I am unsure if this breaks Rule 9., so if its possible please let me know.

Thank you.