Sorry I couldn't be of much more help but I really do advise you to get a new HDD if you're having exactly the same problems. If I had known about this a bit earlier it would have saved me a few weeks worth of headaches trying to figure out what the hell was wrong and how to fix it.
Hey. So I tried a whole bunch of things and ended up fixing my issue. Apparently, my HDD is perfectly fine. The whole issue was resolved by simply downloading the update in Steam's Big Picture mode. I have no idea why this fixed it but it did.