Add local Digital Ocean Volume to Nextcloud as additional storage?

Keeping your data directory in the /var/www/nextcloud/data directory is not the best idea security wise because it could lead to an easier compromise for a hacker (potentially through command injection or some other method). The chances of this being the case may not be very high but still it’s always a good idea to add another layer of difficulty for those trying to remotely exploit your system.

If I were you, rather than adding multiple data directories (not sure if that’s even possible tbh) I would check out the External Storage plugin for Nextcloud: https://docs.nextcloud.com/server/9/admin_manual/configuration_files/external_storage_configuration_gui.html. When going to add storage you can choose local storage and select your DO volume.

You can also mount other shares using the occ command implemented in Nextcloud 9: https://docs.nextcloud.com/server/9/admin_manual/configuration_server/occ_command.html#files-external-label.

Hopefully this gives you a helpful starting point! If you need help totally say so and I can try to start experimenting on my own Nextcloud instance and give you some sort of walkthrough on the commands you would need to use.