How do you use multiple HDDs in Nextcloud?

ℹ️ Support
12

Reading through the documentation it appears you cannot use this as you intent. You will have to assign the external storage to a user or group.
Like explained in post 2

https://docs.nextcloud.com/server/latest/admin_manual/configuration_files/external_storage_configuration_gui.html

I highly recommend to setup lvm (logic volume management) which will require a full backup of you harddisk and reformat to the appropriate partition type.

Lvm is a virtual partition that takes multiple storage devices and creates one working partition of the size of all storage devices combined.

I would suggest reading one of the many tutorials about lvm

Hint. You only need lvm for you cloud storage not your entire system. (Your choice)
If I were you I would take a clean/empty hdd and set it up as lvm then add it as your primary cloud storage, copy all previous data from the old cloud storage then convert the old storage to lvm and add it.