While this idea is interesting conceptually, practically speaking, it's not useful. In order to use this, you need to have dummy files that take up a single byte. If someone deletes a dummy file, the slack space is now eliminated and can be written over at any time. However, this technique won't work on an SSD with TRIM turned on. TRIM actually zeros out the slack space. It's definitely a WhitePeopleThings to do something like this. It's not practical, but interesting, nonetheless. If I wanted to hide something, I'd probably re-partition the drive to make it appear it was smaller or claim there were a bunch of bad sectors when there weren't any and write to the dummy bad sectors.