I changed an old Macbook HD to an external but now it's stuck with Read Only permissions. How can I change it to Read & Write?

A friend gave me an old Macintosh HD he had laying around from a previous computer. I converted it to an external HD using an enclosure kit and used it to store a bunch of old files that I wasn't using. I've recently dug this HD up and for some reason it's set to Read only. I'd really like to store more files on this HD but it won't allow me to. Help?