Comment 4 for bug 727668

I'm not sure how to mark this in launchpad, but this also affects os-prober and thus update-grub2. Indeed, update-grub2 relies on os-prober to find whether there's a Windows partition to add to the Grub menu. However, because it uses os-prober and therefore os-prober can't mount some NTFS partitions (not all apparently), Windows may sometimes disappear from the boot menu in dual-boot systems.

http://askubuntu.com/questions/22698/update-grub2-not-finding-windows7-partition/30392#30392