Taking a sledge hammer to your drive is exactly what forensic data recovery people want you to do. They can recover data from severely damaged drives but they have a much much much harder time to recover data from a drive that has been overwritten even once. The best method is to overwrite once on track center and once off center using firmware like ATA secure erase. There are currently not any known techniques for recovering data from drives wiped in such a way, but spin stand microscopy will pull a fuck ton of data from a drive that is smashed into even a thousand pieces with a sledge hammer.