Assuming your Ubuntu guest VM is on the same network as your NAS, and depending on how you’ve created your share(s), one of these two articles may be of help:
Mounting CIFS + SMB network shares for Plex use
The basic checklist
Prepare the NAS
Create the CIFS/SMB credentials file for use in /etc/fstab
Create a ‘mount point’ directory structure on your Linux machine which will be where all your media shares are grafted and made available for Plex use.
Create an entry in /etc/fstab for each share you wish to mount.
Mount and debug as needed
A. Prepare the NAS.
This is unique to each NAS. In general, you only need grant read permission. Should …
Mounting NFS network shares for Plex use
The basic checklist
Prepare the NAS:
Create a ‘mount point’ directory structure on your Linux machine which will be where all your media shares are grafted and made available for Plex use.
Create an entry in /etc/fstab for each share you wish to mount.
Mount and debug as needed
A. Prepare the NAS.
This is unique to each NAS. In general, you only need grant read permission. Should you desire Read/Write for yourself, we will allow for it
On Lin…
Google may be a better resource for you however regarding the more general topic of how to mount a NAS-homed network share from Ubuntu; it’s not specific to Plex.