Why overwrite a file more than once to securely delete all traces of a file?

Question

Erasing programs such as Eraser recommend overwriting data maybe 36 times.

As I understand it all data is stored on a hard drive as 1s or 0s.

If an overwrite

Answer 1

"Data Remanence" There's a pretty good set of references regarding possible attacks and their actual feasibility on Wikipedia. There are DoD and NIST standards and recommendations cited there too. Bottom line, it's possible but becoming ever-harder to recover overwritten data from magnetic media. Nonetheless, some (US-government) standards still require at least multiple overwrites. Meanwhile, device internals continue to become more complex, and, even after overwriting, a drive or solid-state device may have copies in unexpected (think about bad block handling or flash wear leveling (see Peter Gutmann). So the truly worried still destroy drives.