I never bought an extra standalone hard drive to add more space since I usually have enough space on my PC since I mostly use a PC to browse the internet, play a few PC gmaes, and do casual tasks like type out homework, listen to MP3s, or no task which use a lot of hard drive space where I need to add more space.
Plus, most computers I own now have hundreds of Gigabytes of space which I rarely fill up.
Since getting my laptop around 4 years ago now I have not needed to get an extra hard drive because of space to be fair which is quite a shock as I do have a lot of music on my laptop.