Move Folder from External Drive back into LR

One method I have used in the past is to use the operating system (Finder in your case) to copy that folder where you want it to be. Then, using Lightroom, locate the folder on the archive drive and choose the option to Find the missing folder and point Lightroom to the folder in its new location. After you have done that, if you choose to do so, you can remove the folder from the archive external drive.