Yup, definitely keep backups.
Also, in case you want to be able to search the library (even though the books aren't available) -- you can keep the database file somewhere local and use the environment variable CALIBRE_OVERRIDE_DATABASE_PATH.
All the calibre metadata will be available for you to look at, and you don't have to keep all the books on the hard drive.
Use with caution, and don't change anything while the library is unattached
because calibre might lose track of the books.