If you are afraid someone might get the data forget low formating or any software. Someone that knows what they are doing could get the data that you thought you erased.
There's some machines made just for erasing drives that cost about 3k+, they contain really strong magnets. I guess you could take the drives to a junkyard and see if they will let you use their magnets.
Drilling holes through the drive(s) is probably best Idea I've seen posted here if you can't afford a 3k+ magnet machine, whatever it's called. ;)