Omar, as dfkd says.. always replace the pad set at once. Also ..whenever you replace the rotor, even if the pads look great, they should still be changed at the same time! Last but not least, you said that one side had alot of pad left? I might want to check the caliper sliding pins, infact , go to Autozone or what have you, and buy " calipers sliding pins lube" usually when i get mine, they come in small packets. Anyway, everytime you do your brakes, always lube the sliding pins, the ones with that tiny boot moving the caliper on braket. Over time the lube inside will dry up, or the boot might come off, then the caliper will seize, and one or both side of the pads will fail, along with the caliper, it would be good preventive maintanance. Good luck!!