Installed game on primary drive, but secondary drive hits 100% usage while playing?

This is one of the more puzzling things I've encountered:

I have DX:MD installed on the same drive as my OS, which is an SSD. It was originally installed on the D: but the loading times were way too long and the D: drive was maxing out usage.

But here's the strange thing... the D: drive still hits 100% usage when I'm playing DX:MD. Not only that, the game stutters when the D: hits 100%.

How the hell is this possible? I triple checked that it was installed on the C:, and it only does this when I play MD. Is there some kind of temp file steam keeps on the D: while I'm playing?