I am having difficulty understanding what it is that you have done. Have you imported the original NEF files that are on the external drive? The only way you can get access to any file in Lightroom is to import it.
Sounds like you converted to DNG upon import, and destination was set to internal drive. And so Lr is doing what you said - the DNG is the original image from Lr's point of view, the NEF is, well, no longer important..
If you want to access originals on external drive, make sure original is on external drive then add in place, or make sure external drive is the import destination when you copy or move upon import. convert to DNG or not as you see fit..
Thanks, I am pretty sure the import was done with "add.." and not "copy...". I'll try a test run.
Is there a log someplace of actions taken by LR?
There is a history of your Lightroom editing. But I don't believe there is any record of other actions.