It seems not all firewire drive enclosures are equal. Those with an Oxford 911 chipset apparently work properly with OSX and you can boot off them. The one I got today from Maplin was unfortunately not an Oxford chipset but a Prolific PL3507. It works ok in so far as it mounts the disk in OSX but trying to boot from it doesn’t work – it just hangs and doesn’t find it. Time to find a different one.