First, I know nothing about Macs. And I do not recall seeing any Mac user describing anything similar to you. From your detailed description, it seems to me that your OS and/or your backup software is creating some temporary folders and assigning your catalog files to those missing folders without Elements becoming aware of them. Hence, Elements thinks that the files are missing. So, IMO, it is a Mac issue, not an Adobe one. You have probably figured that out yourself. Frankly, I can't offer you any help.
Thanks for your reply and yes I think it's a Mac problem too and I might look at some Mac help forums for advice. Thanks again Greg.
Just to let you know I think I have solved the problem by disconnected my Seagate Backup Plus Drive external HDD and everything seems ok now. I'm still externally backing up to the Crucial X8 SDD. After a bit of "detective work" I did suspect what the cause was so fingers crossed.