I am obviously missing something very simple
I copied the existing library to the thumb drive, and confirm it has all the books plus a metadata file 3,202 KB file type is Data Base File, plus metadata_db_prefs_backup.json file.
Copied that to " D:\ Calibre Library " , a new folder on the "D" drive HD.
Opened Calibre .....no books listed.....on the library tag, clicked the create/switch option.
Small box, showing " Your current library is located at D:\ Calibre Library" with the options beneath.
Entered the location as D:\ Calibre Library , OK ................result is " D:\ Calibre Library is not an existing location "
Stuck now