Mount is mostly a Linux term for '
make the OS (explorer) see the drive'. In most case USB drive auto-mount.
Windows just does it (they don't call it Mount)
If this is a real Calibre Library (folder).
things got easy.
Make Calibre see the Library (on the drive).
Right click the Library icon: Create/Switch: USE EXISTING: <point the dialog to where it is>: OK
Select books from the flash Library: right click COPY TO LIBRARY: select the target. Since this is a backup, I would NOT delete.
Duplicates are handled by the Setting in Preferences: Adding Books:Adding Actions