So, it's pretty well established that it's almost impossible to permanently, irreversibly erase data from a hard drive, but I do understand that it can be made ridiculously, unimaginably hard to recover.

So what do you think is the best way to do it? Is a clean reformat enough? What programs do you prefer to go a step further than that? I've heard of programs that basically "insert blanks" (as I'm told) to clean a HDD well.

Reason I'm asking is we're getting ready to donate an old computer to a charitable organization out there that just got flooded (lost ALL of their computers). Since where I work is a financial company, however, making sure HDDs that we get rid of are totally clean is a BIG issue.

So what do you suggest?