Rsync local storage folder from Nextcloud

I am new to Nextcloudpi and i have a question about synching my nextcloud data with rsync.
I am running Nextcloudpi within Docker and I have bind two local external storages (USB harddrives), lets call them “Myfiles” & “Backup”.
I access “Myfiles” in nextcloud with the “external storage” addon (local).
When I use nc-rsync my nextcloud data is synched to “Backup”.

So far so good, my problem is: rsync ignores “Myfiles”, the files are not synched. Even if I see them in Nextcloud. Only Nextcloud itself (the configuration files) and the standard nextcloud data files (Documents, Photos, Nextcloud.mp4 and so on) are synched.
How can I sync my external storage “Myfiles” with rsync?

Or another question would be, how can I exchange the standard folder with Documents, Photos, … to “Myfiles”?
Already tried with Docker to bind the nextcloud standard folder “/www/nextcloud/core/skeleton” to “Myfiles”, but it didn’t work (or I did it wrong… wrong path… don’t know).

Please help!