Can you e.g. Save Fortnite on an HDD?
You mean download the game to an HDD instead of an SSD?
Why not?
Can you.
Fortnite won't make a big difference between SSD and HDD either
It works, the only difference is the loading times
Naturally. After all, they are both storage media.
You just have to expect longer loading times, which even get worse over time.