Note to self/anybody else it might help: if you have disks with previous dmraid headers (potentially corrupt etc.), you need to remove the dmraid package before grub can find a Windows partition that resides on those disks. This is because the dmraid package is attempting to map the drives to /dev/mapper locations. update-grub/os-prober not detecting Windows 7 was originally published by Martin Paul Eve at Martin Paul Eve on June 02, 2014.