The easiest thing to do is to copy all your images from the internal HD to the external HD, using the OS tools, by drag-n-drop of the top-level folder(s).
Then from inside LR, in the Library module, Folders panel, right-click on the top-level folders and "relocate" them to the external drive (just like "find missing folder").
After you are sure everything is working correctly, you can then delete the images (and not your catalog) from the internal HD using the OS tools.
Optionally you can also put the catalog on the external drive, but things will be faster if the catalog stays in the internal drive.
You can move folders to the WD MyCloud using Finder by simply dragging to the external hard drive. That will make a copy of the folder and files. Then in the LR Library you right-click (or control-click) on the folder name in the left-hand panel and choose update folder location. Navigate to the external drive location select the copied folder and LR will re-link everything automatically. The beauty of this method is that you can ensure everything is OK before deleting the folder from your internal drive.
When importing new photos, I would suggest you keep the WD connected and in the import dialog choose a folder in the right-hand destination panel to import into on the external device. That means no more moving/copying.
This is great advice. It makes me a little more secure. Will I looks the edits I have in my catalog---since i put the files somewhere else?
OS tools meaning just right click and delete right? I tried to cut and paste then realized it would take too long...my cpu froze and I was lost! ESH
After moving the files outside of LR, you will need to tell LR where the files now are.
In the Library module, Folders panel, right-click on the top-level folder(s) and "relocate" them by navigating to the external drive.