How do you copy ONLY new files?

I'm working on a movie. Every time I capture new footage, I edit it. All the media and project files are stored in a single folder. After the edit session, I like to back up to an external harddrive all the footage I captured and the updated project files.
Is there an easy way to copy files to the external drive without having to either rewrite all the old files or select all the new files by hand?

In other words, what is the easist way to copy only the new files?

Thanks in advance!

 

There are many back up utilities here such as FolderWatch that will do what you want. You don't say if you are looking for a free utility or are willing to purchase a program so I suggest you go through the link to see what specifically meets your needs.