Hey there, g335!
In general, @mrt3 is right about storing files on the HDD and installing apps and OS on the SSD.
But the WD Black is fast enough to let you play games/programs as well without any issues, since it's one of our high-end gaming drives.
Of course, a mechanical drive cannot compare to an SSD though, but you could put the LESS demanding programs on the HDD to save up on the space of the SSD.
At least that's what I'd do!
Hope this helped! Best of luck!
SuperSoph_WD