I used to rely on DS Cloud to backup my complete Android mobile onto my Synology (both internal and external storage). But after Android 4.4.x, the files must be stored under /storage/<your external sd>/Andoid/data/com.synology.dsclooud/files. It means DS Cloud may not backup the DCIM folder locate on the external SD.
The solution is to get rid of DS cloud and use DS File or DS Photo - or a third party, but this a less preferred option for me...
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It has a feature to backup all photos from a mobile (Android or iOS) into one folder/album of the Photo Station. But it means that Photo Station must be installed.
See documentation here (loo for Upload and Download Photos).
It's really slow because it needs to create the thumbnails, etc... for Photo Station and seems even to stop from time to time. You have to open it to check that it runs effectively.
Pay attention to not check the option that frees space. It will deleted the photos on your mobile after the upload.
NB.: It also backup movies ("You can upload photos or videos from ..."), but does not seem to do it as long as all photos are not yet uploaded. Many old videos were not yet uploaded onto my Synology although photos with the same age were already processed. And after the backup of 1000th of photos, I finally saw a long list of videos being uploaded.
It has a feature to backup all photos and videos into any subfolder of a shared drive. As far as I am concerned, I do backup into a subfolder of my "home" on the Synology.
See documentation here (look for Backing up Photos and Videos).
Pay attention to not check the option that frees space. It will deleted the photos and movies on your mobile after the backup.
DS File can backup photos and videos from all detected locations containing media: DCIM (external storage), Office Lens, PhotosEditor, WhatsApp Images, WhatsApp Video. But you may not specify yourself a custom folder. It's nevertheless the best option to backup all media in my opinion.
Attention, I noticed that DS File consumed a lot of power during the first backup, so it was suggested to be put in deep sleep mode. But doing that stops the backup background process. Compared to DS Photo, it's quite exactly the same behavior and configuration but one can chose the target folder on the Synology. Also, it is much faster (as it does not have to create anything for the Photo Station) and backup all videos and photos simultaneously.
There is now a new application, named Synology Moment, which also comes with a Backup feature for photos and videos, similar to Photo Station. I didn't test it yet.
(Synology Moment is combined with Synology Drive, an application replacing Cloud Station Server)