I had this, went through 3 sets of discs and pads in about 2500 miles before I realised both (17") front wheels had nasty flat spots on the inner rims due to pot-hole damage. The only way to spot this is jack it up, put it on axle stands and spin the wheels while you watch the inner rims from underneath the car with a torch. Or easier if you take it to a garage or tyre place with a lift. I got the wheels straightened and refurbished, it needed new tyres too but after a 4th set of discs and pads it has been fine since - about 7000 miles. The flat spot in the rim causes the tyre to leave contact with the road at that point where the flat is. This causes the disc to wear unevenly, possibly the ABS plays a role too. Hard to believe I know, but worked for me. If your front rims have flat spots then your backs are probably similar. Good luck. Matt
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