In the Settings page of Superior Drummer 2 the user has the option to set the primary and backup paths to a library.

Both the Primary and Backup packs should NOT be set to the same location.

The backup path should only be used if you have a 2nd installation of the Superior Drummer (or specific SDX) sound library in a 2nd location. For example, many users have laptops and have all of their full installations of the Superior Drummer libraries installed to an external hard drive. They also have a small partial installation of the library installed to the internal hard drive of the computer. This way, if the external hard drive is not plugged in Superior Drummer 2 will use the backup path on the internal hard drive.