I recently gathered all my photos from multiple external hard drive and put them on a single new external hard drive. So now I have to rebuild my catalog because I want all my photos and the light room catalog to reside on this new external hard drive. What is the best method to do this? Is it"copy as dng, ad, move or just copy
It is a little unclear what you have actually done. You said that you have gathered all of your images from other hard drives and placed them on a single new external hard drive. If it means that you have actually moved those images then it isn't necessary to move or copy the images at all. They are already in their new location. If you have retained the same file structure for the different folders, all you'll have to do is right-click on a folder and update the folder location. If you have moved images around to different folders then it might just be easier to create a new catalog and start over with all of your editing. Can you describe a little more fully what you have done up to this point?
You don't have to "rebuild" your catalog, and whatever you do, DO NOT IMPORT THE PHOTOS AGAIN.
Let me repeat that. DO NOT IMPORT THE PHOTOS AGAIN.
You have to tell Lightroom the new location (on the new EHD) of your folders or photos. Follow these instructions: Adobe Lightroom - Find moved or missing files and folders