Today, the BBC News website reported on a Which? Computing magazine article that claims that the physical destruction of a hard drive is essential to protect the data on it, citing that they had retrieved deleted files from second hand drives.
For most folks in computing, the ability to retrieve deleted files is not surprising, nor is the possible survival of data even after repartitioning and reformatting a disk. Nevertheless, the idea that is is impossible to reliably and easily erase a disk has been contested by The Great Zero Challenge. Quite simply, their challenge is to a data recovery company to retrieve data from a drive after the unix command dd has been used to overwrite the whole drive using /dev/zero as a source.
Given no-one has accepted the challenge, it seems you might not have to reach for the hammer after all.