GB--
A hard drives files if simply erased and removed from the trash bin can still be easily recovered if
all the sectors occupied have not been overwritten. This is done using a compression algorithm known as lossy compression--the parts of the file not available are predictable to a high degree by the pattern that is left. The scenario you describe is somewhat different, i.e. the writing of a file of exactly the same size in the same space. It is a lot trickier, but files pieces do not use every bit in every cluster. Even if eash and every bit, however, is overwritten, there is a residual magnetic charge left from the old image. It is sometimes called a palimpsest, which derives from the time when writing material was very expensive. When the usefulness of the document was over, it would be scraped clean and reused, but it was still possible to rescrape it and recover what had been underneath. Similarly, data written on magnetic media over other data can be "scraped" off to allow access to what underlies it. This can be done because the magnetic particles that are used to store binary data have a memory once written to. It is possible, though expensive and tedious, to go back two or three iterations of data written and overwritten. The only--so far--reliable method of completely erasing data from magnetic media such as hard drives is to write a bit-wise pattern of zeroes many times, effectively reducing the magnetic residual memory to an unusable state. The hammer method would not deter a determined tech with enough funding and at least most of the pieces. Hope that helps! (BTW, just what are you trying to erase?
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