There has been a lot of talk around the fact that even formatting a hard drive does not fully delete the data contained on it, and that said data could quite easily be recovered by sombody with the right tools.
I will be thinking of updating my computer, probably next year, if not sooner, and may want to sell my old one, as a working unit. How, then, could I be certain that data, such as credit card or banking details, personal and business accounts, etc, would not be recoverable from my old hard drive, should I pass it along to another user?