Sorry to hear about that... This is a problem that randomly occurs across all third party NI libraries (not just ours) in older versions of Kontakt.  Native Instruments has become aware of this and has addressed this issue in the rollout of their successor to Service Center - Native Access. Native Access is required if you're using any products that were released with or after Komplete 11 (September 2016). Please check out the FAQ and Compatibility info for Native Access before starting to use the application. Here's how to get your Library back in the browser tab using Native Access:

1. Download Native Access. If you already have Native Access installed, opening the application will update the metadata for every Kontakt Player library.

2. Restart Kontakt. If the problem persists, please contact Native Instruments registration support and provide them with further detail about the specific library that disappears and the steps to reproduce the disappearance. Native Instruments will then investigate and update the database as needed.

If you're not using Native Access, here's the 'old fashioned' way of fixing this issue between Service Center and Kontakt (pre-Native Access):

When you add a library, Kontakt should create an XML file that's used to ensure your library stays in Kontakt each time you open the Library Browser. Recent versions of Kontakt have had problems generating this XML file when users add Third Party Kontakt libraries. Navigate to the following folder:

  • Mac OS: Macintosh HD > Library > Application Support > Native Instruments > Service Center
  • Windows: C > Program Files > Common Files > Native Instruments > Service Center

Here you will find various XML files related to the NI products on your machine. You should see an XML file bearing your product's name, such as EXHALE.xml or EXHALE 2.xml. If you don't, download the files in the links below.

download REV.xml

download Signal.xml

download REV X-LOOPS.xml

download EXHALE.xml

download SUBSTANCE.xml

download ANALOG STRINGS.xml

Then copy the downloaded xml files to the directory listed above (make sure to move the XMLs out of the provided folder that was created when you opened the zip, the XMLs should sit directly in the 'Service Center' folder). Before opening Kontakt again, you will need to reinstall Service Center. You can reinstall Service Center by following this link and download/running the installer listed under NI Service Center:

http://www.native-instruments.com/en/support/downloads/drivers-other-files/

As long as Kontakt can now find the XML files in the 'Service Center' directory, you should no longer find that your libraries are disappearing.