Originally Posted by theducks
Warning: When copying the other Library to your HD. Make it a NEW folder location outside of the existing Calibre Library.
Never, Never place (or delete) files or folders inside the Calibre Library.
There have been a few cases where the library contents (all the folders named for the authors plus the metadata.db file) have been copied into an existing folder having valuable content. That's also bad, as Calibre considers everything in that folder to be its property and will remove it all if you tell Calibre to do that. A "Calibre Library" is a single folder, and you should move that folder, not just its contents. I put all my libraries adjacent to one another (at the same level) and just name them differently. You can name the library folder anything you want, then switch to it.