OK...
re-created the usb stick and found the problem:
When copying library from it's backup location to usb stick all the directories change their owner from whatever it was in backup directory (in this particular case it happened to be 1024) to the root (which I am when doing the copying), but...
metadata.db and metadata_db_prefs_backup.json (and all the files inside sub-directories) do not change the owner and retain whatever it was originally (in this case 1024)
In doing so I have re-created exact issue I have reported before, and exactly as before if I run the script it reports that database is corrupted.
Ownership of the files inside sub-directories of library doesn't matter a hoot and Calibre is not bothered about it at all (once it gets going)
It is only the two database files that seem to bother it when starting
Luckily by now I know exactly what to do
go on the usb stick into the CalibreLibrary and manually modify the owner to root and Bob is your uncle
I am not knowledgeable enough to understand what is going on - I just report
For now I will modify my Library and use it OK.