External Drive - You don’t have permission to upload or create files here

[alfredb]

Good Morning!

The user that is running the web server must have r/w access rights on the mounted disk.

Setting up help required

I’m not an expert on that topic and I don’t know nor have any raspi. But I know, also from the docs, that the HTTP server user must have r/w access rights for any storage attached to owncloud. And AFAIR, access rights always default to root. A look at this page might help you finding a suitable configuration: