[chaley]

How does the content of the metadata file you wish to update differ from the existing .metadata.calibre file? That file contains calibre metadata for books in the folder, and is maintained by the existing driver.

If it is a difference in format, then wouldn't a converter be easier to write?

Edit: If you really want to maintain a second metadata file, then you would add an override of sync_booklists. Write your metadata file, then call the method in the superclass (USBMS).