Thanks, @Nancy ... as you suspected, the “.dropbox_bi” folder disappeared when the backup was completed.
But it's not just me that was worried about this. This shouldn't happen because the files are already on the external drive, just like on the local drives, and shouldn't have to be duplicated!?
In any case, I'm glad the whole thing went so smoothly.
Unfortunately there is no reference to a clear solution in this thread. Are everyone affected now satisfied? How did they solve the problem?