File permissions problem, NC 13, Ubuntu 16, Nginx webserver has no access - reboot temp fixes problem

Recently my data drive had some problems… I was able to use some drive utilities and fix it. I restarted NC and everything worked! Then… some hours later I had an error message that the webserver did not have permission to access the data directory. Sure enough, the owner was root so as root I ran these commands:

chown -R www-data:www-data satastorage
find satastorage/ -type d -exec chmod 750 {} ;
find satastorage/ -type f -exec chmod 640 {} ;

I rebooted the system and it all worked… then it stopped with the same message.

Any ideas?