I've got some old models that were produced in C4 Pro. They won't open in Pro 5 ( I/O error) as well as back in Pro 4 (I/O error). Both versions are loaded on same computer. ( if that is partofthe problem) I'm in a load of hurt if I can't open these files. They represent hours of blood, sweat, and tears!! If anyone has a solution I would be very much obliged! Thanks.