Erasing Old Hard Drives Need Help

I just got some computers from in front of a mental health office. I found out that they worked, and am going to sell them for parts. Problem is, I know there has got to be some pretty sensitive data on the hard drives. I know that formatting isn't enough, but shredding all the files on even 3 passes (the min) takes 10 hours for a 20gb. I'm thinking about formatting and then using a neodymium magnet set to scramble everything. The magnets are insanely powerful, they'll wipe a floppy from one and a half feet away. I figure I'll reformat after the manget treatment. I was wondering if this would do a good job on the drives, cause if not, I have a friend who works around MRI machines. (If you haven't figured it out yet, I can't afford a copy of evidence eraser).

Any other ideas on wiping drives?

Hey, gimme a break, I'm a photographer first, animal rescue driver second, then computer nut. I try to stay up to date so forgive me if I ask a stupid question.