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The principal used in obtaining these files that have been erased goes past that of simply the pointer to the file being removed. Though we think of hard drives as digital devices that store a magnetic charge in each bit, in reality a magnetic charge is more like an analog device. By carefully studying the flucuations in the charges, it can be ascertained what bits were stored there in the past. This technique only works so far back in the past, but it well proven to retrieve data that has already been destroyed once, perhaps twice. Beyond that it becomes difficult to chart which state the bit has been in.

What this means is that if you desire data security, you need to use something like this professional degausser which generates a strong electomagnet field to completely erase everything on a tape or disk. The other way to completely scrub your hard drive is to format it over and over again. There are some utilities which can write alternating patterns of ones and zeros in order to test a drive's these work, too, but in either case you must make many passes. One time will not do.

Reminds me of something I witnessed back in the days when an IBM AT was a very fast machine, floppies were actually floppy and Symphony spread-sheets displayed green on a black screen...

An overpriced desk jockey in our work group was having big troubles with her PC; she couldn't get it to access some vital files. The computer "guru" came and spent at least 20 to 30 minutes trying to extract her data. When he was unsuccessful he handed her the floppy. She was fuming and criticizing his incompetence as she placed the disk back into its usual storage location: on the side of her file cabinet held in place by a magnet.