Hard Drives to be destroyedFormatting should be the first step regardless of whatever operating system or computer type.  There is a big debate over the algorigm required to securrly remove evey piece of data from your drive.  I recommend grabbing free programs like Eraser.  Eraser works by “overwriting it several times with carefully selected patterns.”  At this point, unless you work in a specific industry requiring ridiculous standards, your disk is TOAST.  Further, “…a single overwrite using an arbitrary data value will render the original data irretrievable even if MFM and STM techniques are employed.”

Just to be sure and have some fun, you can physically damage the disk with a drill or hammer.  Paying people to take them away and professionally destroy your hard drive is normally too costly for noncommercial pursuits.