You don't state which OS you have but if it is Windows than you might be able to do what you want. Open a cmd window and use the xcopy command with the /T option. Now there are some limitations to this command. I suggest you look at the help for the xcopy command. that is: xcopy /?. It will not copy empty folders and it will not copy sub-directories so if your structure is : X:\birdname then you should be ok but if it is: X:\birds\birdname it may not.

The OP said he wanted to copy the directory structure and not the images. Drag and drop will copy everything including the images.

I tend to agree that it doesn't make sense to make a whole new parallel set of folders merely for disk space reasons. Equally, it makes no sense to have a new Catalog per drive. All that would be tedious to do and then doubly tedious to maintain, for example if a new folder is required in both places. Lightroom's Catalog can work just as easily with images stored on one drive, as on another, or across both.

So moving perhaps half of the present set of folders, assuming there is some sensible way to divide them (even if that is just A-M, and N-Z) onto the new drive, would allow you to continue using them there, with space to add more images into the same folders (alongside the present ones) and managed by the same Catalog.

It would also allow you to continue using the ones that remain on their current drive, with space now to add more images there, again managed by the same Catalog.

If in future some of these need to move around again to other drives (say, you buy a setup large enough to store it all) then that is no problem.

Or if the new drive is big enough, all the images and their folders could move there... which would then need a minimal and trivial change so far as your current Catalog is concerned (a couple of clicks to re-browse the parent location of the entire "tree" full of birds). Everything carries on functionally either side of this kind of a behind-the-scenes change.

Analogy: the actual nature and current location of the money in my bank account is of no real interest to me, so long as money's available when I require it, and so long as I have confidence that good practice is happening which will avoid the bank going out of business or refusing to let me have my money in the future. The bank is in fact moving this money around constantly; lending it, investing it and so on. And by virtue of my relationship with the bank, all kinds of services become possible, which would not be if I just stacked banknotes under the mattress. The account balance on the ATM screen - or to return to LR, the fact that the image still works - is what matters to me the customer.

RP