Nextcloud docker external storage

Hello,

I would like to use a Intel nuc with a 120GB SSD and a 1TB USB-HDD as nextcloud server. I would like to run nextcloud in docker and so I tried the example file with nginx and lets encrypt.
My problem is now, that everytime I try to put my data folder on the HDD a get an error. I’ve I let the compose file as it is it works.
is there any possibility to put the data on a external HDD?

On the NUC I have Ubuntu 18.04 runnig.
here is my docker-compose file:
version: ‘3’

services:
db:
image: mariadb
command: --transaction-isolation=READ-COMMITTED --binlog-format=ROW
restart: always
volumes:
- db:/var/lib/mysql
environment:
- MYSQL_ROOT_PASSWORD=xxxxxx
env_file:
- db.env

app:
image: nextcloud:apache
restart: always
volumes:
- nextcloud:/var/www/html
- /media/data/nextcloud/data:/var/www/html/data
environment:
- VIRTUAL_HOST=xxxxx
- LETSENCRYPT_HOST=xxxx
- LETSENCRYPT_EMAIL=xxxx
- MYSQL_HOST=db
env_file:
- db.env
depends_on:
- db
networks:
- proxy-tier
- default

proxy:
build: ./proxy
restart: always
ports:
- 80:80
- 443:443
labels:
com.github.jrcs.letsencrypt_nginx_proxy_companion.nginx_proxy: “true”
volumes:
- certs:/etc/nginx/certs:ro
- vhost.d:/etc/nginx/vhost.d
- html:/usr/share/nginx/html
- /var/run/docker.sock:/tmp/docker.sock:ro
networks:
- proxy-tier

letsencrypt-companion:
image: jrcs/letsencrypt-nginx-proxy-companion
restart: always
volumes:
- certs:/etc/nginx/certs
- vhost.d:/etc/nginx/vhost.d
- html:/usr/share/nginx/html
- /var/run/docker.sock:/var/run/docker.sock:ro
networks:
- proxy-tier
depends_on:
- proxy
volumes:
db:
nextcloud:
certs:
vhost.d:
html:

networks:
proxy-tier:

and a ls -l of the folder /media/data/nextcloud/

drwxrwx— 3 www-data www-data 4096 Aug 24 21:31 data

and the log of nextcloud
{“reqId”:“xxxx”,“level”:3,“time”:“2018-08-24T19:31:01+00:00”,“remoteAddr”:“xxxx”,“user”:"–",“app”:“PHP”,“method”:“GET”,“url”:"/apps/files/?dir=/&view=extstoragemounts",“message”:“file_get_contents(/var/www/html/data/$
{“reqId”:“xxxx”,“level”:3,“time”:“2018-08-24T19:31:01+00:00”,“remoteAddr”:“xxxx”,“user”:”–",“app”:“PHP”,“method”:“GET”,“url”:"/apps/files/?dir=/&view=extstoragemounts",“message”:“stat(): stat failed for /var/www/html/$
{“reqId”:“xxxx”,“level”:3,“time”:“2018-08-24T19:31:01+00:00”,“remoteAddr”:“xxxx”,“user”:”–",“app”:“PHP”,“method”:“GET”,“url”:"/apps/files/?dir=/&view=extstoragemounts",“message”:“stat(): stat failed for /var/www/html/$
{“reqId”:“xxxx”,“level”:3,“time”:“2018-08-24T19:31:01+00:00”,“remoteAddr”:“xxxx”,“user”:”–",“app”:“PHP”,“method”:“GET”,“url”:"/apps/files/?dir=/&view=extstoragemounts",“message”:“stat(): stat failed for /var/www/html/$
{“reqId”:“xxxx”,“level”:3,“time”:“2018-08-24T19:31:01+00:00”,“remoteAddr”:“xxxx”,“user”:”–",“app”:“PHP”,“method”:“GET”,“url”:"/apps/files/?dir=/&view=extstoragemounts",“message”:“stat(): stat failed for /var/www/html/$
{“reqId”:“xxxx”,“level”:3,“time”:“2018-08-24T19:31:01+00:00”,“remoteAddr”:“xxxx”,“user”:”–",“app”:“PHP”,“method”:“GET”,“url”:"/apps/files/?dir=/&view=extstoragemounts",“message”:“touch(): Unable to create file /var/www$
{“reqId”:“xxxx”,“level”:3,“time”:“2018-08-24T19:31:01+00:00”,“remoteAddr”:“xxxx”,“user”:”–",“app”:“PHP”,“method”:“GET”,“url”:"/apps/files/?dir=/&view=extstoragemounts",“message”:“touch(): Unable to create file /var/www$
{“reqId”:“xxxx”,“level”:3,“time”:“2018-08-24T19:31:01+00:00”,“remoteAddr”:“xxxx”,“user”:”–",“app”:“PHP”,“method”:“GET”,“url”:"/apps/files/?dir=/&view=extstoragemounts",“message”:“touch(): Unable to create file /var/www$
{“reqId”:“xxxx”,“level”:3,“time”:“2018-08-24T19:31:01+00:00”,“remoteAddr”:“xxxx”,“user”:”–",“app”:“index”,“method”:“GET”,“url”:"/apps/files/?dir=/&view=extstoragemounts",“message”:"Exception: {“Exception”:"OCP\\Files$