Data Recovery Systems and other companies make a good living recovering
data from computers that have been in fires, floods, and accidental
erasures, and they are frequently hired by law enforcement agencies to
recover from intentional erasures--even sophisticated ones. A single
"bit" on a hard drive comprises thousands of flux changes, and each time
it is written, the boundaries are a little different, and the intensities
are a little different. The techniques are all very well known and well
reported for decades. All you need is a reader that can produce a map
of the flux intensities about 100 times finer than the read head itself,
and good software will tell you what the last dozen or so written bits
were at each location, and analyzing the positional offsets can tell
you which bits were written at similar times.
If you don't believe it, just use one of those programs, then call
them up and pay them to recover it for you. Won't be cheap, though.
-- Lee Daniel Crocker <lee@piclab.com> <http://www.piclab.com/lcrocker.html>