theres only one possibility if one pad is wearing faster than the other and thats that u have a caliper sticking. the fact that you are hearing a grinding noise at low speeds or when u apply brakes could signify this but i would get the tire off and check the pads see how they ook. or the noise could be as tegbooster said the little wear strip on the pad itself. and if that is the noise and the inner pad is the only one really wore that says u have a sticky caliper.

to fix this just go down to the parts store and pick up a new caliper or rebuilt depending on what is available and throw it on along with new pads and rotors. problem solved. oh yea and bleed the brakes if u didnt think of that. only way to really fix a sticky caliper is to replace it.