Comment 8 for bug 570765

The GRUB delay is a direct consequence of os-prober not finding any other operating systems when run from within ubiquity.

(We think that this may be down to a migration-assistant bug, which leaves the other filesystem mounted in such a way that os-prober can't easily get at it from within the installed-system chroot - at least not without double-mounting it rather than bind-mounting, which I'm not sure is safe.)