Thanks for all your help up until now! I don't really know much about HDDs, so I don't know how to handle this. All I want to know is if I bought a new hard drive, how would I add it as another partition or possibly merge it with the already existing one?
Just plug the new drive into a free SATA port and make sure the port is enabled in the BIOS. Assuming you're running windows, go into "disk management" to initialize, partition, & format the new drive. Assign it a new letter and you're good to go.