Thanks for the advice Kirk - but I seem to have gone backwards!
I had been storing my Pictures folders on the iCloud Drive which is on my MacBook Pro hard disk, but on opening PM6+ again found nearly all my folders had been corrupted with multiple duplicate images, along the lines of Image 1 2, Image 1, Image 3, Image 4, Image 5, Image 6 etc. - where 1,2 and 3 are the originals and 1 2, 5 and 6 etc. are the duplicates.
I've spent hours cleaning up the folders to get back to 1, 2 and 3 and have moved the Pictures folder as a whole from my hard disk's iCloud Drive to the Macintosh HD under my user name.
Then went to Catalog Management and did an update, but it seems to have identified every image on my Macintosh HD not just the folders I had in the 'Favorites Pane'.
To get out of this mess can I delete the PM6+ Catalog folder and files as a whole, and then start again by importing the cleaned up folders into the Favorites Pane, and without losing all the keywords attached to the individual images?