You could put use one library with all the book details in it. Then archive the books elsewhere and store a reference to the book location. You could probably use an identifier with a file URL for when the drive with these books are connected. Or a custom column with it. Something like this is used by people to refer to non-book references.
As a NAS was mentioned above, if you have one and it can run calibre, you could put the archived books on that and access them via the calibre server. If you did this, the reference could be a link to the book via the calibre server.
And for any bored developers out there, that is screaming for a plugin. Have function to send the book to the server using calibredb, add the reference details to the local library, and then later fetch the book using that reference. Then some functions to update metadata on the server if you change it locally. "Easy" for any developer now that I've written the requirements
