Can't move folders to external drive - Mac

To free up internal drive space, I need to archive the last 6 months of work - roughly 20k photos organized in dozens of folders by month/day and project for a client. A pretty complex file system I have to keep intact.

Each month's folder is structured like this:

January

     01-01

          Project 1

               Capture

               Master

               Output

          Project 2

               Capture

               Master

               Output

     01-02

          Project 1

               Capture

               Master

               Output

          Project 2

               Capture

               Master

               Output

...etc

I've added the external drive to LR and created a folder for 2017. When I try to drag the Folder from January (located on my internal drive) into the newly created 2017 folder (on the external) from within the Library module, the new folder is highlighted when I hover over it. But when I let go to complete the drag & drop, the folder bounces back to the original position. It's like LR is rejecting that move. However if I try to move just the "Capture" folder, it'll take it. I can't go through and manually recreate that entire structure, it would take hours.

Any explanation or work-arounds for something like this. I really need to both clear up internal hard drive space AND keep my file heirarchy intact.