Fixing permissions for a local attached external drive

I am a newbie to Ubuntu, NextCloud and this forum. Recently installed NextCloud on a Mac Mini running Ubuntu 20. Installed NextCloud and is running. I would like to use the attached thunderbolt drive as the main drive for NextCloud but all I was able to figure out with the web’s help is how to add it as an external drive. Even as an external drive, I am unable to upload anything to the drive through the admin’s account or any account I created. I either get the Forbidden error or Operation blocked by access control. Any tips how to go about fixing this?
Appreciate any guidance.

The data folder and all contents must be owned by the www-data user.

Changing the data folder location is easiest to do on a new installation (although you certainly can move the data folder later).