The reason is that linear editing programs will read the meta data on the internal recordings and know that it's DATA RANGE that needs to be interpreted. On external recorded footage, that metadata doesn't exist and so they interpret the prores footage as LEGAL RANGE, which -- as Chapman says -- " This results in the recording levels being transposed into incorrect levels for processing. So when you add a LUT it will look wrong, perhaps with very dark shadows or very bright over exposed looking highlights. It can also limit how much you can grade the footage."
How does FCPX handle this and how can we make sure that FCPX sees it as DATA RANGE?