You won't get a harddrive to erase easily by magnet, Trust me, its nearly impossible w/o spending some big cash.
You have three options:
One - call the vendor, let them know you have sensitive material on the drive and wish to physically destroy the drive before sending it in, per Dept of Defense standard practice. Tell them the drive will not pass wipe protocol 5220.22. They may let you once you mention DoD. If they say yes then drill holes straight thru the drive in the platter area. This is what they do with some of the drives at my work.
Two - destroy the drive and eat the cost. a new 500gb is $120 here. An 80gb'er is $40 on Newegg.
Three - trust the manufacturer not to be wasting time trying to recover your files justt o see what you have.
98 SSEi 111,222 miles.
Intense PCM, LS6 Valve springs, PEMS, FWI. Hawk Street Pads.