Do not disassemble a hard drive. If you disassemble it, it will pretty much become useless because tiny particles of dust will collect on the platters and can interfere with the read/write arm.
Avoid using a magnet...it most likely won't do anything unless you have a large, extremely powerful magnet.
Just use one of the tools that I suggested (KillDisk, DBAN, or GParted). We have a guide for GParted on our site, located here: http://www.techist.com/forums/f127/g...torial-224994/
It is a free tool, and only requires a blank CD to burn the image file on.