Brake Dust
There has been a lot of talk about it on other web-sites. The rear wheels on almost all the models (with ABS or not) have been building up a lot of brake-dust while the front wheels look pretty clean most of the time.
The groove problem (and Mazda does know about it, but has not issued a TSB on it yet), is said to be either the calipers not fully releasing or that there was a slew of rotors that had a slight "raising" of serial numbers right on the disk (extremely hard to see) and that was causing the pad and the rotor not to "marry" evenly while braking.
Some cars are worse than others and some have no problem at all (luckily mine) with the grooving. Although, my left rear disk did start to groove a little bit when I first bought it and strangely enough has stopped and the disk looks very good now. BUT!!, the brake dust???, oh yea, I have to wipe down those rear wheels every day to keep them matching to the clean look of the front wheels.
