There are three groups of files you need to care about:
- original image files: you can place these anywhere you want, internal drive or external drive or network drive.
- catalog file: you can place this anywhere you want on a local drive, internal drive or external drive; location of image files is independent of this.
- smart preview files: these are located in a folder tree in the same location as your catalog file.
If you want the smart preview files on the external drive, you need to move the catalog file and the previews folder and the smart previews folder to the external drive using the OS tools (Finder or Explorer).