If all you want is to erase the hdd then just format them with the filesystem of choice. Since you don't care from a security sense then the format will provide you with a clean slate. This assumes the partition scheme is usable as is. If not you will need to 'fdisk' or 'cfdisk' the device(s) partition scheme.
You may want to use 'Qtparted
' or 'Parted Magic
' to do everything in one application. I also suggest that you use a IDE-USB interface as this will facilitate the operation for so many drives.
The above links and others can be found at 'Slackware-Links
'. More than just SlackwareŽ links!