Based on the Adobe recommendations for optimizing Lightroom and Photoshop performance, there are 2 sets of files that benefit the most from a fast disk:
I believe this is the best disk arrangement that meets these recommendations with the fewest disk drives. Do you agree?
C: - SSD - operating system & programs
D: - SSD - Lightroom catalogs & previews, Photoshop scratch disk
E: - fast, large conventional drive - image files
One of my requirements is that the non-technical user NOT have to move files (for example, images) from disk to disk on a regular basis. (Otherwise you might also put this month's "active" images on the SSD drive, for example, and then archive them monthly.)
The Adobe recommendations come from these two articles:
There is no real advantage to having a separate SSD for the catalog and previews. And there would be no real benefit of having current month images on an SSD and then moving them to the conventional drive on a monthly basis. I have my programs, Lightroom catalog and previews all on my system SSD drive with images on internal and external conventional drives. Lightroom performs very well.
I can't speak for photoshop, but with regards to Lightroom, you don't need speed on the disk where you store the photos. You will never notice the difference in Lightroom between a fast disk to store the photos and a slower disk.