see video at youtube

In this video I will show you how to back up your Faces data base.

first go to your account folder than go to the "Pictures" folder, there you will find a fill called "iPhoto Library" right click it (control click if you don't have double click mouse or trackpad) and press "Show Package Content", you will find a lot of files "DON'T MESS WITH THEM" just copy the files "face_blob.db" and "face.db".

Now paste the files in your external HD
Enjoy