Mystery D: drive is blank

I am not positive that this is the right place for this, so feel free to move it if needed. OK, I have a Windows 8 Laptop (model should be in my profile), that I use as my main computer. It has a 1TB hard drive on it. When I go into my computer, the C: drive only shows about 372 GB of space total. There is another hard drive listed as D: drive. Now the way I understand this is that D: is...