The above procedure should help you to add only new and approved files in your catalog. The pictures files are still themselves in their old original drives, which have to be plugged in to be accessible (otherwise they are considered 'offline' in the catalog)
You'll probably want to copy or move those files to your present computer.
This has to be done from inside the organizer.
My advice : don't move them into your present folder. Create another folder or a subfolder of your present catalog. That way, the media files you'll move by drag and drop in the folders view will go to an empty folder. There will be no risk of collision with and already picture file with the same name; the Windows explorer would not like that.