Data on external drive

I am trying to get Nextcloud up and running on my Raspberry Pi4. The Site actually hosts, hurray! However I’d need to have all the data and files stored onto an external drive, but I can’t get that work. Portainer tells me, the Volumes are bound to the right places. The Nextcloud App tough allways tells me it’s got around 50gb of Space left, which is the SD Card the System is running.
That is in my docker-compose file:

- /external/1TB/nextcloud:/var/www/html

I also tried to have just the Data Folder external, but it doesn’t change the results tough.

- nextcloud-data:/var/www/html
- /external/media/1TB/nextcloud/data:/data

Can someone help me? I think I’m stuck. Thanks!

Edit: Oh my god, I’m just a big bloody Idiot. I didn’t set up permissions correctly for that stupid drive! Well, my bad. I suppose this is solved now.

I had a problem similar to yours and I managed to solve it by following the steps described on this site.