I have a question about mechanical disc brake pads. I have a second hand electric bike (Volt "Pulse"), and it is my first experience of disc brakes. I have found that the pads fitted initially worked well, but developed a load squeal. I failed to cure the problem with by tightening screws and cleaning the rotor with Isopropyl alcohol. Recently the brake lost power so I removed the pads and discovered that they have worn unevenly, with what appears to be a part of the pad not in touch with the rotor (see photo). Is this incomplete wear normal? Why would it be occurring? Is there a cure? Thanks.