when was the last time you made a backup to it? you could try making a update to your online backup first like adding a small txt file or jpg and then attempt download it again may could allow you download the whole thing without error.
Its updated every day, usually around 1 gig. That takes 4 hours or so. My upload speed is much slower than my download speed. I understood that restoring from an incremental backup would take a little longer. But thought I could get by not having a local backup.