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.
Why use a partition? In part so the software can work its recovery magic on your primary drive (i.e. C:) without overwriting itself, and in part so users are less likely to accidentally delete it.
My advice to you, Gilbert: Leave your D: drive alone. If it has a few extra gigabytes of available space, you could probably house some data there -- but why risk it? If you need more storage, pop in a flash drive or external hard drive.
On the flipside, if you don't envision ever needing those recovery tools, you could always wipe the D: partition and absorb the space into your primary drive. Want more info? Read PC World's
Contributing Editor writes about business and consumer technology. Ask for help with your PC hassles at , or try the treasure trove of helpful folks in the . Sign up to have the e-mailed to you each week.