I had a spare 522 a couple years ago that I cracked open to see what was inside. It contained a Maxtor 120GB hard drive that I yanked out and put in my computer. There's nothing special about it that makes it a "video drive." It's the exact same EIDE hard drive that you would purchase for any computer with an EIDE interface.
That being said, I've never replaced a hard drive in a DVR but I remember reading about people who had experimented with it. If I recall correctly, a new drive will format itself to work in your DVR but it will only recognize a pre-determined capacity, regardless of the actual drive capacity. In other words, you cannot increase your storage capacity by installing a larger internal drive.