OK, it turns out that the second drive had once been part of a RAID as was not being recognised. Using: sudo dmraid -E -r /dev/sda as described on this page: http://ubuntuforums.org/showthread.php?t=2003675 fixed it.
** Changed in: ubiquity (Ubuntu) Status: New => Invalid -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1330172 Title: Unallocated space on one of two drives not offered for installation To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/ubiquity/+bug/1330172/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs