Have you tried opening calibre from a command line (or using a batch/script file) with either the --with-library or --library-path option?
Under Windows, the command line would look something like:
Code:
"C:\Program Files\Calibre2\calibre.exe" --with-library D:\Calibre_Library\Cantrip
Please note that this will set the default library to Cantrip the next time you open calibre.