This is not our return/warranty policy, but Danner Mfg's policy. Every manufacturer handles their warranty differently, and this is how they choose to do so. Mag's are actually pretty good pumps, however things do happen, which is the same with anything you buy, wether it is a new car, or a pump for your aquarium.
You would need to send the entire pump to them. More than likely they will issue you a new unit, but I do not want to speak for them.
A grounding probe will work for small amounts of voltage going into the water, in which more than likely most MJ powerheads and other submersible pumps, as well as Mag's, put off into your system. This type of stray voltage is extremely small, and most times is not felt by us when we put our hands into the tank. This probe will prevent deformities, hole in the head, and other problems that fish would incur from prolonged stress of small amounts of stray voltage. However, when a substantial amount of voltage is put into a tank it is very common for a grounding probe to not ground it out. I have had my hands in a system that was having 40volts leaked into it, in which the system had not just one, but three grounding probes and I still got zapped fairly well.
Keep in mind that it is also highly recommended to use a GFI outlet that in most cases will trip when a pump goes defective. This is not always the case, and every defect is different, so sometimes even a GFI will not trip.