I'm betting that the two internals run in parallel, and they drain equally at the same time. I’m sure this could be easily tested with the right equiptment. I can't immagine they would switch from one to the other. Assuming they are in parallel, the full one will charge the dead one until they are both equall. In my case I don’t think it was a bad battery, but either not a full charge, or the full hard drive. My drive is chock full now, so I should do another test and see if I still get the 6.5 hrs. I’ll do it in the next few days and post the results.
Matt