I have Elements 15. When I try to drag one image to another folder I get this message - "Folder not available", if you have a network drive ensure driver connected. I have done this ok for ages but now won't let me do it. I can have some success with the "move" command but not always. Also when I want to load new images into an existing folder it opens a new folder with exactly the same name which is rubbish.
Sometimes I get a message my database is broken (sorry can't remember the correct phrase) reinstall but everything generally works ok
I could download Elements again I suppose, would I have to uninstall first
I know nothing about Macs so can't really comment further, other than to say that I have seen reports of problems using Time Machine as a backup on an external drive.
Some Buffalo external drives are what are called NAS (Network Attached Storage) drives. This means they can be accessed from any computer on your home network (if you have one). This is different than a simple external drive which can usually only be accessed by the computer to which it is attached. I don't know whether this is part of your problem or not. Hopefully, somebody with more knowledge about Macs than me can provide additional information.