I'm using an SD card as a temporary main storage. I could technically put in an ssd, but I lost/broke the part where a screw would go to hold it in place (it's a strange laptop…), so the only option I would have is to tape it, and I don't feel comfortable letting it potentially flop around in there… As for what a swap file is (that's the name I found for it), it is virtual memory. It was reserving that space to use as RAM if need be, when I already have 2/3rds as much ram as storage.