Unfortunately, the nature of technology make is nearly impossible to erase the data beyond recovery, at least not without the specialized tools (such a Hard Drive Eraser is). You can always try to physically destroy hard drive, melt it, or degauss it using a strong magnet, but neither is really practical, particularly for companies that wants to get rid of a dozen of hard drives every year. When you delete a file, or format a partition or drive only a reference links to a names of files and folders are removed, actual data is still sitting intact on a hard drive. In order to clean the actual data from the drive you have to use a drive erasing software (or disk wipe software as sometimes is also referred to). Erasing hard drive works by overwriting sectors with random patterns of binary 0's and 1's. A bits of information can still be preserved after drive erasing but not only it would take a highly specialized and expensive procedure but the random nature of such collected information would make it nearly unusable.
Hard Drive Eraser is free tool that helps users preserve their privacy by securely erasing hard drive volumes and partitions. To download Hard Drive Eraser please click here.