Backup/sync Mac photo to external hard drive It's already making me crazy - as a newbie with Mac. I have problem with backup/sync of my pictures.
I would like to backup pictures from Photo library (Photo app) to external HDD but more like sync. 


All the new pictures in my Mac should be copied to external HDD.
Pictures that are already on external HDD should NOT be copied there twice (not classic backup of whole library).
Pictures I erased from Mac should NOT be erased from external HDD (not classic sync).
If import of new pictures would be automatic when external HDD is connected it would be great (not manually copying new pictures).


Is that possible with Mac? With Windows I used to do just import new pictures to external HDD library. It never deletes any picture from external HDD even I delete it from my PC library.
Have you tried a sync/backup tool like ChronoSync (which I highly recommend)? It can be scheduled to sync to when your external drive is mounted. It also does a great job of handling package files (which is what your iPhoto library is). It's from Econ Technologies and can be trialed for 30 days.