OK...
so here is what I have done:
Scenario:
I have a list that needs to be saved to SD card for retrieval via USB at a later stage.
App Inventor side of the solution:
1. You need an Activity starter that will launch the app that writes the data in a text file on the sd.
2. You need the package name of the "Eclipse" app.
3. You need the Class name of the Class that is being used to write the file.
4. You need a string to pass to the "Eclipse" app
5. You need to set an ExtraKey (has to be the same as in Eclipse)
6. You need to set an ExtraValue (any string)
Eclipse side of the solution:
I attached a text file with the Java code
If you all can wait a few hours I will post a full demo with app inventor blocks :)