iOS Streaming Assets inaccessible

I'm trying to read a number of files within a folder structure under StreamingAssets. This works fine in the editor, and the files are copied to the Raw folder with my xcode project. However, when I run on the device, I get "Access to "/var/mobile/Applications/xxxxxxxx/dev.app/Data/Raw/MyData/SubDir1/Foo.bin" is denied." Not sure what's wrong.

  mainDirectory = Application.dataPath + "/Raw/MyData/";
     
  string[] directories = Directory.GetDirectories(mainDirectory + "SubDir1");
     
  foreach (string dir in directories)
  {
     string[] files = Directory.GetFiles(dir, "*.bin");
     for (int i = 0; i < files.Length; ++i)
     {
         DoSomethingWithFile(files[i]);
     }
  }