I haven't used a scratch drive for years.

If you continually (on a daily basis) write a lot of data (>20GB) I'd consider using the HDD for that purpose, you'd wear out the SSD prematurely.
Most SSDs are "tuned" for let's say 10-20GB per day, giving the 5 year or more life expectancy.

So basically, if you write less than 10-20GB per day I'd use the SSD and forget about the scratch drive.
Having said that, if you do a lot of sequential writes you might as well use the HDD, it's just as fast or faster than most SSD's. (less wear, same speed)