How do I format an external hard drive for resale?

I did the simple “right click and format” method but I read online that there is advanced software out there where people can still recover files? Is this true? If so, is there a safe way for me to sell the hard drive? It is a Seagate GoFlex Desk with Replica auto backup software. I just want to sell it for extra cash because I don’t need it but want to make sure everything I deleted off of it cannot be recovered. Thank you!