I've seen people have Orthos on HDDs since they are bigger than SSDs for a cheaper price. I have a 4TB SSD so currently I have everything on that - mainly to get it out of the way and help my multi-level backup programs. Depending on the brand of SSD, you may have also got specific maintainence software from the maker that lets you do it (both Samsung and Crucial have this). Modern defraggers should recognize you have an SSD, warn you in big letters about doing a defrag if you try to start one (you can always over-ride it and do one, since it's your PC and they are just a program, not a cop - please don't do one) and only really run a TRIM on the SSD. It helps to even out the wear on the SSD.

This is the equivalent of clearing out the Master File Table and getting rid of old entries that are no longer valid (file deleted or moved) and releasing that cell for further use. With an SDD, the only thing you need to do is TRIM. I am moving a lot of files around because of things outside of X-plane and my frag levels can shoot up. I still believe in defraging HDD and I do that every so often on my PC.

Ultimate defrag