different brake discs?
 

it appears that on my S the rear wheels always have more dust brake particles than the front ones, are the pads different all together or is it just the compound or a different ratio of pressure between front and back, it seems that the discs are the same on all corners.

Not completely sure but front and back pads are from a different OEM supplier...
Mark.

The rear pads are usually organic (i.e. not semi-metallic) depending on whether or not a PO replaced them with something different. The reason is so much of the stopping pressure is on the front, it's not as hard on the rear brakes. To keep the rear rotors from being worn down quickly, organic pads are used.

Another reason you notice a big difference may be a PO who replaced the front pads with low dust pads.

Btw, the rear discs are a little smaller in diameter than the front, but the thickness is the same. Our front rotors lasted 62K miles, but we still have the original rear rotors at 73K.

so if the fronts work more that the rears should have more dust on front and not rear?
what kind of organic compound are we talking here, i know that asbestos was remove front brake discs a few years ago so what do they use now?