I had to recently buy another external drive since both I had are almost full.
• my laptop has 60gb, almost full before I bought my 2nd hard drive, mostly with family photos & videos, SimCity 4 downloads, music... currently I moved almost everything to my other Hard Drive, so it's only 12gb used for programs to keep laptop running and another 8gb for recent downloads or moving photos & videos from my cell phone/3DS XL before organizing on one of my external hard drives.
• had another external HD 500gb, which got full after a while with more family photos/videos, tv show & movies, lots of Pokemon related stuff. Before buying my 2nd hard drive, it was 490gb full. Currently only 50gb is used for music. I deleted lots of files I didn't need, mostly some I had 2 or 3 copies of.
• mid March, I bought another hard drive, a 2tb drive, moved majority of family photos/videos, most of my TV Shows and Movie collection on it, my entire Sim City 4, and lots of other junk stuff I have gotten along the years.
Laptop: 60GB - used up: 20gb
1st Hard Drive: 500GB - used up: 50gb
2nd Hard Drive: 2TB - used up: 750gb