Can you try mounting the filesystem in question, unmounting it straight away, and then running os-prober? If this works around the problem (I'm not suggesting it as a permanent solution), then this is the same as bug 683355.
Can you try mounting the filesystem in question, unmounting it straight away, and then running os-prober? If this works around the problem (I'm not suggesting it as a permanent solution), then this is the same as bug 683355.