While creating a new folder (or a subfolder) within Lightroom Classic is straightforward enough (click the plus icon in the Folder panel header and select Add Folder or Add Subfolder), what many people don’t realize is that this is also the way to make Lightroom Classic aware of an already existing folder on disk that has yet to be imported and, may even be empty. Lets say for example, you have created a folder using the operating system on an external drive that you want to move images to, yet Lightroom is not aware of that folder. To add this folder to the Folder panel click the + (plus) icon on the Folder panel and select Add Folder.
Instead of creating a new folder, navigate to and select the desired folder. If the folder is empty, Lightroom will simply add it to the Folder panel. If there are images in the folder, Lightroom will display the Import dialog so that you can decide what to import, what metadata to apply etc..