I recently decided to split my main library in to 3 sub libraries.
I made 3 full copies of the library folder, including all sub-folders/metadata etc. renamed each of the main folders to better names.
Loaded up calibre, pointed to one of the new sub-libraries, then deleted all the books I didn't want it that library, repeated with the other two.
All the columns etc were copied.
Was easier than setting up 3 seperate libraries then copying/moving the books from the main library.