[ddouglass]

Do you want to move them all from C to the external drive folder?
If so copy them over to the new location. Verify they copied and then delete them on the C drive.
Then run the merge database in Hoster changing/ deleting the C drive folder from the list leaving only the external drive listed. then run it. That is all there is to it.
If you are leaving the files that are on C drive where they are, then just run the merge database with both folders.

Running the Merge database does not move any files. What it goes is change the BookID assigned to those that have been listed as being duplicate.