OK thanks - my mistake. I should mention that it appears that Nero has some sort of 16GB .nrg limit or something - when I created an image of the 50GB blu ray, it created TempImage.nrg, TempImage.nrg.001, and TempImage.nrg.002, all of which were around 16GB. If I go to copy that image directly through Nero's software, it finds all three files and makes it appear to be a single 48GB file.
Looking at the nrg2iso source, I see it's simply skipping the first N bytes of the file. I also saw a suggestion to use the mount command directly on the .nrg file with an appropiate offset. Neither of these worked, presumably because each individual .nrg file is not itself a valid file. Do you happen to know if I were to create a single file which was all but the first 307,200 bytes of the first file followed by all of the second and third files if this would be a valid ISO file?