How to permanently remove data from external hard drive?

I have this external hard drive that i no longer need and want to p on to someone else, but I am not sure how to permanently delete files on it. When I right click it, it gives me a "format" option. Will that do it? Do I need some kind of software. Is there any kind of freeware I can use for that? Note that this is about an external hard drive, not the hard drive on my computer.