[cctalk] Mac SE disk cleaning

Passing on a request: anyone know of a way to erase user data off a Mac SE hard disk that will leave the disk and Mac OS intact? The machine works and could go to a new home but has privileged medical data on it so just deleting files isn’t sufficient — the free space has to be overwritten. Are there any utilities available on floppy disk that could do it? Plan B is taking a sledgehammer to the drive, but seems a waste to go right to euthanasia if there’s another option.