Quote:
Originally Posted by Chunk-O-Funk  You can download free utilities that will overwrite free space on your HD many times over making it virtually unreadable except for those with the knowledge on how to recover it. |
This is only sort of true. When someone takes a disk that's had all of its bits flipped and restores data from it, they are able to do so because the magnets on the disk aren't exact enough to push the physical positioning of the bits to "true zero" or "true one". They get somewhere in the vicinity, which is close enough for the heads to store and get that data. It's not close enough for someone with the right forensics
hardware to be fooled and reconstituting the data is easy at that point. It's sort of like with audio cassettes, how just recording stuff over already recorded stuff doesn't quite work. You have to record blank space over the top once or twice and
then record your stuff.
If you flip every bit on the disk to "1" and then back to "0" and back and forth several times, eventually the alignment of the bits will be such that even with an electron microscope you couldn't take the platters out and reconstitute the data. And at the point that someone is using that level of hardware to find out what's on your disk, the money is better spent elsewhere on different kinds of surveillance.
Usually what is done though is to take the output stream of some random number generator and write the results to the disk several times. This makes the bits sit at 1 and 0 in random order rather than in an order which reflects where they sat previously. You do that on a disk a few times and, if your entropy pool is sufficiently large, it's impossible to reread the data using today's technology.
Nino, a decent data wiper is DBaN:
http://dban.sourceforge.net/
When I was working as a sysadmin at the US EPA, we had to physically destroy any disk we retired and document the destruction procedures we took. Some of those were really fun to write up. Many of the magnets on my kitchen fridge are from those old hard disks.
