Wipe

Wipe: "Tuly deleting information from disks is very difficult. When data is written, the write does does completely overwrite the underlying bits. There are a number of reasons for this, but primarily it is because the disk heads do not write precisely to the same location every time. Another reason is that signal strength and media sensitivity vary over time. Finally, the error correcting code on disks which helps prevent corruption also makes it easier to recover the underlying signal of previously written data.
A simple explanation to consider the scenario when writing a '1' to a disk. If there was already a '1' on the disk that is being overwritten, the the signal strength will be slightly higher than if there had been a '0' that was overwritten. With modern technology, it is possible to go down not only 1 'layer', but often '2' layers of overwritten data. For this reason, the DoD recommends overwriting data at least 7 times in order to 'wipe' it from the hard-disk.
Secure Deletion of Data from Magnetic and Solid-State Memory "