Comment 2 for bug 1217933

Why might it be a mdadm problem?
All of the md devices are up and accessible from the 64-bit OS, and the /etc/mdadm/mdadm.conf file appears to match the output of the command ' mdadm --detail --scan' suggesting it is correct. Even if I mount the previous 32-bit systems root partition (that just works if I click on the device in nautilus) there is still no sign of update-grub recognising that has an OS installed on it.