code: protected function computeFileSystemDifferences()
      {
          $entityPathByFilesystems = [];
          foreach ($this->resourceManager->getResourceConfigurations() as $resourceConfiguration) {
              $className = $resourceConfiguration->getEntity();
              $entityManager = $this->doctrine->getManagerForClass($className);
              if (!$entityManager instanceof EntityManagerInterface) {
                  throw new \UnexpectedValueException("No manager found for class {$className}");
              }
              $repository = $entityManager->getRepository($className);
              if (!$repository instanceof ResourceRepositoryInterface) {
                  throw new \UnexpectedValueException(
                      "Repository for class {$className} must implement ResourceRepositoryInterface"
                  );
              }
  
              $paths = $repository->getPaths();
              $adapterReference = $this->adaptersByResourceType[$resourceConfiguration->getCode()];
              if (array_key_exists($adapterReference, $entityPathByFilesystems)) {
                  /** @noinspection SlowArrayOperationsInLoopInspection */
                  $entityPathByFilesystems[$adapterReference] = array_merge(
                      $entityPathByFilesystems[$adapterReference],
                      $paths
                  );
              } else {
                  $entityPathByFilesystems[$adapterReference] = $paths;
              }
          }
  
          foreach ($this->adapters as $adapterReference => $adapter) {
              $existingPaths = [];
              foreach ($adapter->listContents() as $metadata) {
                  $entityPath = $metadata['path'];
                  if ('.gitkeep' === $entityPath) {
                      continue;
                  }
                  $existingPaths[$entityPath] = $entityPath;
              }
              $entityPaths = $entityPathByFilesystems[$adapterReference];
              $this->extraFiles[$adapterReference] = array_diff_key($existingPaths, $entityPaths);
              $this->missingFiles[$adapterReference] = array_diff_key($entityPaths, $existingPaths);
          }
      }
docs: Compute de differences between what's in the storage system and what's in the database
  
  @throws \UnexpectedValueException