Back Left Rotor Half Worn?
 

I've had my 2002 Mazda Protege for a couple years now and been all over with it. It works amazingly well and has been low maintenance. Lately, I've been noticing sounds like squealing and metal on metal. They're not super loud, most people in the car with me don't even notice if I ask if they hear them, but gradually they've built up. It seems more at low speeds.

My first thought was that the break pads had worn out, but I still have break pads. They're still quite far from the wear line. Certainly still quite a bit of pad left. I went out and bought them and took the whole thing apart, only to figure out that I still had quite a decent amount of pad. (They're hidden behind some metal so it's kind of deceiving.) But it did reveal something interesting with my rotor.

I was searching online and can't find any other pictures of anything like this, where half of the rotor is worn. I know it says warped rotors are a myth, so I don't think my rotor is bent or anything. You might have to see what I'm talking about, so I've included pictures.

Attachment 8743
You can see on the left of the image, the rotor is still relatively smooth, while on the right side it's heavily worn. That's what I mean, the rotor is half worn and the other half is still smooth.

Here are my brake pads so you can see I definitely still have them. It's hard to see because they're behind the metal, but trust me there's still plenty of pad there.
Attachment 8744

I'm wondering if anyone here has ever seen anything like this before. Obviously I need a new rotor, but if there's something else broken I would also like to fix that, so I don't burn through rotors. :P

This is only one side that's like this, the other side has a perfectly smooth rotor:
Attachment 8745

If you need more images or information I will gladly provide. Any ideas how this could have happened?

Replace both rear rotors, replace all rear pads,lube up the caliper slides, bleed the brakes, adjust & check handbrake, you should be good to go.