Any external hard drive connected to your computer will show in the files panel. You can add files to your library from an external drive just like you would do when importing from the internal hard drive of your computer.
If the external hard drive is not connected when you run Serato DJ any files added from this drive will not display in your library.
The library information for files on your external drive is stored on the external drive itself. This means if you add tracks on an external drive to your library and then plug that external drive into another machine running Serato DJ, the crates and tracks will automatically be visible in the other machine’s library.