I followed this guide to install Nextcloud on my Raspberry Pi 3:
(no matter it’s in German, I hope you can read the commandlets )
But is there any reason to make the local external storage for a primary directory?
My girlfriend (she makes a world trip at the moment and needs a big cloud for the pictures and videos to participating me ) and me would thank you very much.
if you store the data directory into your SD Card. You have limited space there, and the SD Card is not the best choice for high frequent changes / access.
Or you mean the nextcloud folder /var/www/html/nextcloud?
May not be of any use but I had an older installation running on my Raspberry Pi that used my Synology NAS for the data directory.
I got mine to work by setting up NFS on the Synology and symlinking the folder on the Pi to it through my fstab - you can do the same in Windows/Mac/Linux etc depending on what you want the external device to be. I wouldn’t suggest a USB stick or drive plugged into the Pi in case it unmounts or gets corrupted (of course that means another machine on all the time but far safer than USB or SD) - also mariadb is a good idea for your database
When I set it up it was www-data:www-date and 777 then changed to 755 (or 744, cant remember exactly) once I checked it was working.
Maybe it’s of help