Ok. Before I found it'd only be ~200 MB I thought that maybe it'd be continously writing to storage, which I thought was going to wear a hole through it pretty quickly (rewriting that same 200 MB for an hour means you've written to it... 240 times.
Another option is to just store the data needed to re-render the past 15 minutes. But that'd require the game engine to be written to support that.
Also, re-rendering 15 minutes of gameplay footage would probably result in a ~15 minute pause when you decide to save.
So yeah, it's probably continuously writing to storage.
I think drive failure is probably going to be common on the PS4 in a few years, but... IDK. Having a hard drive fail on a game console seems way more acceptable to me than having it fail on a computer.