Hi dj_paige I realised that the Macbook air wasn't happy with the files because it threw up the error msg 'The files appear to be unsupported or damaged' and slowed right down.
As for the opening in LR4 i wanted to double check that things were ok with what i had shot, just a cursory glance.
I guess what i'm really asking is, is there software i can use to open the corrupt file?