You can buy a disk sanitizer or get it for free - this will nullify all data on the disk. There's a lot to choose from. DO NOT format and just call it a day - formatting does not erase files - they are recoverable (that's why some people don't panic and jump to conclusions if they format a wrong drive or partition) - there are utilities that will recover all of the data (or most if there has been some write activities) but anyways, use a disk sanitizer.