Just as an FYI, you should make sure you don't have Calibre open and using the same dropbox files at the same time, they can step on each others toes.
Make sure you only use Calibre on one machine at a time. Calibre only reads the database file at open then saves it again on close so changes made on one machine would be lost if you then closed Calibre on another machine.
This actually applies to anyone who shares their Calibre library among multiple machines.
Just a friendly heads up so people don't lose data
