Wow. First of all, that is a whole new level of documenting-obssession.
My actual edvice would be, try to consider how neccessary this file really is.
About sorting, I have no idea if this will work or not, but you might want to try to build an Enumerator that returns the data directly from the Hard Disk (not saving anything but few pointers maybe), and then trying to use LINQ's OrderBy, which returns IEnumerator as well, which you, hopefuly, can Enamurate and save directly back to the disk.
The only question is whether or not OrderBy saves anything in the RAM.
