Cheap Bulk Eraser for Hard Disks?

cute-boy asks: "Recently I had to replace some hard disk drives from the same batch which had failed, while still under warranty. Because the drives were no longer recognized by the SCSI controller, it was not possible to erase the data on them. In view of the sensitivity of the data contained upon them, and the chance this was still forensically recoverable, our company decided to buy new drives rather than risk the disclosure of their contents by returning then to the supplier. How would you non-destructively (physically) destroy data on a hard disk without access to a bulk eraser? Obviously in this case it's a bit late to be thinking of using encryption."