As you have a laptop, I would say that there are not two physical drives in there. This would make it heavier. There is probably one drive in there but it is "partitioned" so that it acts as two logical drives.
To put your data on it, you will need to manually put some of your files onto the drive, by simply moving them over. So right click on a folder that you want to move, choose "Cut". Go to "My Computer", open your "D drive" (double click) and then right click on the blank space and choose "Paste". This will move the files to your D drive and remove them from your C drive, freeing up space on your C drive. You can also use the drive by choosing to save files to your D drive, rather than C drive, for example when you save a word document, click "My Computer" and then "D drive" and it'll save there.
If you right click on the "D drive" in "My computer" and then choose "properties" it will show how much space is actually left on this drive. It could be a very small recovery partition, with essential data on it incase your computer goes wrong incase you need to reset your laptop back to when you first got it.
Hope this helps!