There are three primary reasons this error occurs:

If you used a custom location (like an external SSD), navigate to that folder in your File Explorer or Finder. You should see folders labeled or "Midi" . If these folders are missing from the directory you’ve selected, the error message is technically correct. 2. Update the Path in Toontrack Product Manager

By default, Toontrack installs its core libraries to these locations: C:\Program Files (x86)\Common Files\Toontrack\EZDrummer Macintosh HD > Library > Application Support > EZDrummer Sweetwater Advanced Fixes Topic: MIDI library in Superior 3 missing !? - Toontrack

If the internal settings fail to save the path, you may need to manually update the system's record of where the files are kept: :