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

Imagine a sector of data on the physical disk. Within this sector is a magnetic pattern (a strip) which encodes the bits of data stored in the sector. This pattern is written by a write head which is more or less stationary while the disk rotates beneath it. Now, in order for your hard drive to function properly as a data storage device each time a new magnetic pattern strip is written to a sector it has to reset the magnetic pattern in that sector enough to be readable later. However, it doesn't have to completely erase all evidence of the previous magnetic pattern, it just has to be good enough (and with the amount of error correction used today good enough doesn't have to be all that good). Consider that the write head will not always take the same track as the previous pass over a given sector (it could be skewed a little to the left or the right, it could pass over the sector at a slight angle one way or the other due to vibration, etc.)

What you get is a series of layers of magnetic patterns, with the strongest pattern corresponding to the last data write. With the right instrumentation it may be possible to read this layering of patterns with enough detail to be able to determine some of the data in older layers.

It helps that the data is digital, because once you have extracted the data for a given layer you can determine exactly the magnetic pattern that would have been used to write it to disk and subtract that from the readings (and then do so on the next layer, and the next).