Is there any diepi program that can do the following:
I upload my iPhone photos through a synology app so I have a local backup on my nas.
I have also a pie with a connected external drive, I would like this drive to be a second backup.
Is there a diepi program that can clone a folder when it detects changes?
I know synology nas has some clone program but that requires both disks to be connected to the synology nas.
I found diet pie- sync, seems like it might fit, but it does not seem to detect changes, rather sync folder once per day, can this be changed to weekly/montlhy?
I also saw some other dietpi sync programs, are the a better fit for me?
No this is not possible via our application dietpi-sync
Which programs you mean?
syncthing and some other
this is not a DietPi own application. It’s a 3rd party product we just include to allow easy installation. If I’m not mistaken, it require a client app running on both side, source and target.
ok, and the diepi-sync does not actually detects changes right, just copies folders and filesonce per day, it does not overwrite files? I have a photo library with many gigs, so I only want never files to be copied.
What I would like I an automatic copying of files from folder1 to folder2 when there is a new file on folder1. But not all files
Usually rsync should take care on this and copy changed files only.
rsync, same as dietpi-sync?
dietpi-sync is a shell script creating the gui aso. At the end it will trigger rsync tool to move/copy files. 
What about using dietpi-cron to change this?
As I read it in /etc/cron.daily/dietpi the dietpi-sync is similar handled like dietpi-backup. The selection of the execution time of dietpi-backup is described there: DietPi tools - DietPi.com Docs.
I guess it basically should also work for dietpi-sync.
But if a daily backup is not “quick” enough you might also use the dietpi-sync part in /etc/cron.daily/dietpi and put it within an own /etc/dietpi.hourly script (I would not recommend doing this via /etc/cron.minutely…).
the aim of dietpi-backup is to backup the whole system and not keep stuff in sync. And to sync a photo library on daily basis should be fine.
Syncthing would make this setup very easy I think. You can install Möbius Sync on your iPhone, which is the iPhone version of the Syncthing client. Then you can set up your photos to automatically sync when you are on WiFi, or on your home LAN, or whatever–no need to bother manually uploading anymore if you don’t want to.
The Syncthing client for DietPi is available in the dietpi-software TUI menu as mentioned, and there is a Syncthing client available for Synology as well from their package center (Synology Inc.).
The benefit of this setup is you wouldn’t have to do anything to keep the files in sync. Syncthing automatically detects when there is a new file and syncs it for you.