Perhaps I don’t understand …
You will redirect the Nextcloud “data” Directory to an external disk? I can’t see, for what reason, nor knowing if it is possible.
I also use Linux Mint 19 build as a LAMP-Server but in a virtual Environment (VMware) with an underlying Win7 Host for basic purposes (DynDNS, remote VNC). On this Host, that SSD and outside the virtuall Maschine I have some shares, that I can mount from Linux. Doing this, you have to create a mount-point (or more) and then rewrite with the correct syntax in the table, located at /etc/fstab
`With sudo mount -a the shares will be activated without reboot linux.
Then you can get access in Nextcloud bei configuring the external storage “local” with the path to this share.
But … This share an its content ist “read only” by default. Up to now I have not found a way to make it writable in Nextcloud.
I take another road to have a writable directory outside Nextcloud. Because I am using VMware, there ist the possibility to define “Shared Folders”. In this way folders on the SSD even outside Nextcloud an outside linux. With the correct VMware-Tools installed in linux, these folders after restart will be mouted at /mnt/hgfs
And that folders are writable by default, but you can set them to read only by configuring VMwares “Shared Folders”. With Nextclouds WEBDAV you can access these folders from everywhere. I use this feature to backup some Content at my Android-Smartphone with the APP FolderSync.