HDD Partition

Hi all.
I have a Seagate 160gb hdd.
I had reason to go into disc management just now and noticed that 21.06gb of the drive is unallocated.
I meant to split it into two but never noticed that the d drive was only 49.86gb.
In the back of my mind i have heard of this before but can't remember why it happens, can someone enlighten me?
The other thing, without third party software is there any way of adding this to drive d or would i need to format d and then mingle the two?
If that is the case i will make do with a third partition!
Cheers.

 

If you want to change your partitions Gparted is the app to have:

http://majorgeeks.com/GParted_Live_d5757.html