Coping new files to "external storage" (local) uses also main (internal) storage

I am using NC 15, and I created a new sync, Therefore I imported an external storage. I can use it with the GUI.
But when I use it with my Windows syncing client, also the original “internal” storage is used. When this is full, the sync stops with errormessages.
I can see, that on the “internal storage” a folder named “update” is used.
Can anyone help me, why this happens? My “internal” storage (VMware --> Debian) is just a few gigabyte. My “external” storage is much bigger.

Nobody has a hint for me ?