Cool thanks RET! So when the primary one is full it'll just start using the other one?
No, it doesn't work like that.
Instead, you'll choose this other drive as a location for your downloads, music, other personal files.
Browser settings, you can designate a folder on that drive as the default download location.
Personal files...whatever application you use, you can "Save to..." or Save As..." to some folder on this other drive letter.
Then, with most current applications, it will suggest that location (on the D drive) as where to save the next file to.