Trying to move files from Mac to Seagate external hard drive

I've read a few posts talking about the need to either re-format my external hard drive or download third-party software to support my Mac having write access to my Seagate external hard drive. I am not super tech-savvy, so anything you provide as a comment would truly need to be in pretty basic language - but what is the best option - re-formatting or the third party software? To re-format, is there a way to maintain the files currently on the drive (I have a significant amount of pictures on there and do not want to lose them)?