That is your secondary drive partition. What does C says? How big is C drive?
I think the whole hard drive would be a 250GB and they have partitioned it into two drives. The D Drive is there to store your documents which in case if your vista crashes, atleast you'll have your documents saved into D drive.
The partition drives are created, in case of emergency you have format your C drive, your data will still be safe as the system will not format the D drive (unless or otherwise done deliberately).
Think of it as having another hard drive in your PC.
Do I make sense here?
Oh and try saving something on the D drive and let me know.
Just like I suspected, yes you can save the files there and infact test it. Save a word document on it and re-open it by going into the D drive and open the file and once you are happy, you can start saving files there