Completely erasing an ext. HDD

Hello,

I have a 1 TB ext hdd, and for some time my friend was using it.

Now I got it back and before using it I want to erase it, but I am concerned that it may have some harmful software that installed carelessly that can mess with my computer, or my privacy for worse. So the question is, how can I erase that HDD without letting it inside my own OS X account, and my folders ? Maybe in a guest account that has no access to anything in my admin account? (I may be over thinking but I am concerned about my privacy 🙂 )

Thank you.