bohrhe

Notice the grooves on the rotor and pad? I wonder what caused them and how to avoid or minimize them in the future.
Initially I thought some hard particles in the pad removed more material on the rotor surface and dug deep grooves. Upon closer inspection it turned out the grooves (channels) are actually on the pad and the marks on the rotor are ridges (extra material left). Just curious how the perfectly round ridges and grooves were formed and how to avoid them on new rotors and pads in the future.

stokesey

I believe the rotors and pads wear somewhat unevenly due to imperfections in the pad material. Once your rotors are scored like that you need to "turn" them or replace them. If they're not making a ton of noise you can wait a little longer to repair it.

csmeance

Imperfections in both the disk and the pad material, but in addition road debris! Any grit/dirt/dust/debris that gets trapped between the pad and rotor can also cause these. You can avoid it by using slotted rotors which cut the face of the pad upon braking for a smooth and new pad face. I run slotted rotors on my car and love them. Downside is the life of pad is cut down by 30-50% as it's getting cut away regularly.