To reinforce what others are saying disks are different from RAM. They are NOT just 0s or 1s. There are residual images. Think of them as being partly analog rather than truly digital. 1 and 0 on a disk are really 1ish or 0ish.
This flies in the face of everything I think I know about digital. It's either on or off, plus or minus, north or south, zero or one. A miniscule grain of silicon is charged or not. No in between.
When a disk file is "erased", the disk space allocated for it is declared empty in the disk's table of contents, or file map, and that section of the disk is available for writing new stuff. Once new stuff is written is that space, the old information that had been there is gone, forever.
In normal computing, all this housekeeping is invisible to the user. But to someone trying to recover an old file that was "erased", i.e., its name removed from the table of contents, the file can be recovered if the info has not been overwritten. Sometimes, the information that resides in a header file of a file may have enough information for sophisticated software to reconstruct the data that had been there by making guesses based on data handling conventions.
When an entire disk is erased or reformatted, it often simply means that the headfile, the table of contents and the bad sector maps of that disk have been set to zero or rebuilt. It doesn't necessarily mean that the entire disk surface has been written over. Often an entire disk can be reconstructed if not much new data has been written on it.
If however, a user specifies a secure erasure, one that literally fills every space with zeros, no data con be retrieved. There is no residue form the previous data, no almost zeros or not-quite ones. There are only zeros, ones and dead spots (kept track of in the bad sector map of the disk).
In digital there are no palimpsests of which I am aware. I look forward to being corrected. I've grown used to falling into the gaping holes in my belief systems.
Dan Scanlan _______________________________________________ pen-l mailing list [email protected] https://lists.csuchico.edu/mailman/listinfo/pen-l
