This is late, but I thought I'd post this here in hopes that it helps someone else...
I had been getting the dreaded medium write error for some time. I tried an external drive (an old one, not entirely reliable) and got the same thing. I thought for some reason it might be software related, so I restored my computer (a bit extreme, and an unlikely solution but I needed to do it anyways, so why not?). That didn't fix it. I did the PRAM reset (which would have no bearing on this anyways, but again why not?). I was about to give up and chalk it up to a faulty drive and bring it to apple to fix under my apple care.
As I was going to leave, I had one more idea. I went to work (I work at Best Buy) and picked up a CD laser lens cleaner disc. My MBP wouldn't load the disc, but when you put it in, the disc spins and tries to load, running the brushes over the laser.
A few times through that and my computer is working like a champ.
So before you try all that other crap, go out and spend the 10 bucks for the lens cleaner. : )