Without actually knowing the size and contents of your drive, I can only make a guess -- but I'm pretty confident it's the right one.
Your D: drive is not actually another hard drive, nor is it the letter assigned to a memory-card slot. Instead, it's most likely a partition of your primary hard drive, a separate area created especially to hold certain files or data.
In other words, you have just one physical drive, but it's divided -- partitioned -- into two chunks.
So, what's on that second chunk, a.k.a. D:? The most likely answer: system-restoration files placed there by the computer manufacturer.
See, few modern PCs come with recovery discs, instead relying on far more convenient recovery software loaded right on the hard drive -- and stored on a special partition.