User can see all other data

Hello, i have a clean install of nextcloud and it work fine. I installed it on a Odroid HC1 with 16Gb SD-Card and a 500GB SSD. So i add the external storage as admin and give it the name “/”, so that the external storage is for everybody and so the SSD got filled and not the SD-Card. The Problem is now, that all users i create, see the documents/files… from all other users. Can somebody please help me? thanks