To move a library from one 'active' calibre installation to another I do this:
- copy the library on computer A to a thumb drive;
- backup the libraries and configuration on computer B
- copy the library on the thumb drive onto computer B - taking care not to inadvertently overwrite any existing libraries;
- add the library to computer B's calibre installation via Switch/Create library.
By 'active' I mean a calibre installation which is in use - i.e. it has existing settings and libraries that must be preserved.
BR