Macbook Pro Automatic Specific Folder Backup

I am currently looking for a way to make it so that I could back up just a specific folder to an external hard drive. For example: if I wanted to have just my Documents folder backup to my external hard drive automatically when I hook it up (kind of like how Time Machine works).

Time Machine won't work because I already have it backing up my main hard drive to my Time Machine partition on my external hard drive. The folder I want to be backed up is on the hard drive in my caddy in place of the optical drive.

So basically what I'm asking is is there a way to automatically synchronize two folders on different hard drives?

I have a Macbook Pro mid-2012 running Mavericks.