[ubuntu] g-io-error-guark Ubuntu's "Backup" fail

I backup weekly to an exteal SATA drive. For the past several backup days, I get errors. In the past I have rebooted and resolved the immediate backup fail. But as this is now persistent, I want to know how to fix this, please.

Full message reads:

Backup Failed

Giving up after 5 attempts. Error: g-io-error-quark: Error opening file: '/media/mark/80cb5313-35a0-432b-9250-4cc62d79a3c8/duplicity-inc.20160528T201042Z.to.20160603T203335Z.vol2.diff tar.gpg': Read-only file system (21)