http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html

In conventional terms, when a one is written to disk the media records a one, and when a zero is written the media records a zero. However the actual effect is closer to obtaining a 0.95 when a zero is overwritten with a one, and a 1.05 when a one is overwritten with a one. Normal disk circuitry is set up so that both these values are read as ones, but using specialised circuitry it is possible to work out what previous “layers” contained. The recovery of at least one or two layers of overwritten data isn’t too hard to perform by reading the signal from the analog head electronics with a high-quality digital sampling oscilloscope, downloading the sampled waveform to a PC, and analysing it in software to recover the previously recorded signal. What the software does is generate an “ideal” read signal and subtract it from what was actually read, leaving as the difference the remnant of the previous signal. Since the analog circuitry in a commercial hard drive is nowhere near the quality of the circuitry in the oscilloscope used to sample the signal, the ability exists to recover a lot of extra information which isn’t exploited by the hard drive electronics (although with newer channel coding techniques such as PRML (explained further on) which require extensive amounts of signal processing, the use of simple tools such as an oscilloscope to directly recover the data is no longer possible).

삭제 방법은 Guttmann, DoD 5220.22가 있습니다. 예전에 진짜 이런 oscilloscope로 파일 삭제가 되네 안되네 따지는 글을 본 적이 있었지만 보고 그냥 잊어버린 이유로 그건 패스. 하지만 논쟁점인것은 사실인 듯 합니다.