At 06:43 PM 1/21/96 -0800, Peter Monta wrote:

Tim May writes:

...

Much more expensive would be various electron microscope-based imaging methods to directly image the domains and extract subtle signs of past write cycles.

I recently took a tour of Park Scientific, the scanning-probe microscopy people, in Sunnyvale. One of their demo-stations showed a small portion of a hard disk (taken with an AFM tip fitted with a small magnet to generate the force). Most impressive. (I did look closely at the edges of the track, but saw no sign of previous writes.) Peter Monta pmonta@qualcomm.com

While I admit that I'm not particularly familiar with modern hard disk head design, I think it is futile to look for data in this way. If they "tunnel erase" the edges of the data track, even small misalignments will not allow remnants of data to remain. (And I assume that "all" modern hard disk drives employ high-precision data-read feedback mechanisms to maintain track alignment down to the submicron level... thermally-sensitive stepper motors and linear positioners of the 1980's are (or at least should be) gone!) Further, modern read-channel techniques (PRML; partial response, maximum likelihood) bring the normally readable signal closer to the noise level than ever before, and the PREVIOUSLY written signal is that much more difficult to resurrect. As a method for gathering intelligence on anyone, I think that this is dead and buried. However, I _still_ want to see brainless operating systems like MSDOS changed to erase (zero) allocated data buffers before and after use (and especially before re-use!), so that parts of vital files don't accidentally get written to the ends of other files.