Do you get any messages?
Doe you have full write access credentials on both of the folders?
No, I get no messages. Yes, have full write access.
From where to where are you dragging the photos? Are you dragging a single image from Grid view to a new folder?
What is your OS and LR version?
.. and make sure you are dragging from the center of the thumbnail, not the edge.
.. and make sure the external drive is connected, because you can see the preview thumbnail even with a disconnected drive.
I'm dragging within LR from a folder to its subfolder that I created. I've done it a million times in previous versions. And I'm dragging from the center of the thumbnail.
OS 10.9, LR CC 2015
Thanks so much.