The Plex installation process under DSM 7 will place the app and data folder on the same volume.
If the app & data folder had been on separate volumes when running DSM 6, the installer would mention it. That would be Scenario 1 in the Install & Migration steps (linked below).
If the installer did not mention it, then the app & data folder were already on the same volume, and you would follow Scenario 2.
