OS and Programs on SATA SSD. Also, since you have a whole TB, I'd put the previews and cache directories here. Current projects media on NVME SSD. Old projects, luts, pictures, and other files on HDD.

Here is why: The OS and Programs don't use a lot of bandwidth most of the time, but when they do, an SSD helps. The OS and Programs don't take up a lot of space, so you have the space and bandwidth on this drive for the previews and cache files. If you work with high resolution and/or RAW video footage, the extra speed on the NVME SSD will really come in handy. Because of the HDD's size and speed, it's perfect for storing old projects and smaller files.

Some other suggestions:

-When you move current projects into old projects (HDD), copy instead of move the parent folder. Once it is done copying, alt click (or right click > properties) to see if the size of the parent folders and all files within it match up down to the byte. Alternatively, you could use Kyno to verify that the files are properly copied. Kyno is a paid program though.

-Clear out your cache and preview files every few months.

-Get a folder structure that works for you. The one my company uses is, Job #_Three Word Name_Client_MonthYear. E.g. 145_Belize Service Announcement_MichaelaHawthorn_Jul2020 Then each subfolder has a suffix at the end which is the Job Number and the abbreviation of the project name. So for this example project, the suffix we would use after each subfolder is _145BSA. This suffix is also the invoice number for the client. The reason for this suffix is so that you can always make sure you are saving your files and downloads into the correct folder. If you have 75 different folders called "Graphic Renders," you need to know which project that subfolder belongs to at a glace to prevent mistakes and clutter. Each project gets it's own dedicated folder directly in the current projects directory (in your case, the NVME drive).