Hmmmm. Can't be sure but you said you're playing it on an external HDD? I've heard there's a cutscene file in FFXIII that's larger than 4gbs and files larger than 4gb can't run from an external HDD. look into it a little further, but like I said I'm not 100% sure if that's the issue.
"We will FIX, the UN-FIXABLE!!! just ehh.. lemme take a nap first.."
PS3 Slim 120gb --> 3.55 Kmeaw --> Multiman --- BUT NOT ANYMORE
(Sold this system after updating to 3.56)
PS3 Phat 60gb (BC) 3.61 OFW (still waiting on a new exploit