Access files on a USB drive using As3 in Android

I write applications using Animate/Air/As3. I'm having trouble using As3 to access a USB thumb drive that's plugged into an Android tablet. I'm not sure what the native path to the files on the thumb drive might be, and the get volume calls don't appear to work. I can get the root directories, but that doesn't tell me where the USB files would be either. What I would need is a simple apk that can read a file from the USB drive. I can then take the source and incorporate it into my code.