There is an unused m.2 slot. I was considering adding an additional large, slower drive later on. It’s mainly to keep the OS as snappy as possible. It also means the black Friday discount kicks in (requires a Samsung drive). Amusingly, I work in broadcast TV, so working with uncompressed video does happen.
My research into the 980 Vs the 990 is that the sequential r/w is only a little slower, but random r/w takes a significant hit. I might just bite the bullet and adjust to a 2tb 980 for €69 more.