FWIW -- Conventional wisdom, from almost the beginning, says FSX should reside on it's own, dedicated hard drive. I won't quote the AVSIM FSX Guide because it's copyrighted, however the AVSIM Software and Hardware Guide for FSX was assembled, and is maintained, by Paul Johnson and is available to members on the AVSIM Forum. If you read Nicks *FSX Bible* over at SimForums it's covered in even more depth. Nick, as abrasive as he sometimes is, is still the ultimate FSX Guru -- all things considered. (my opinion)

Ideally, you should have 3 drives - one for the OS, one for FSX and one for all the other stuff.

Too, I have also read, many times, that a mechanical drive should not be filled beyond 60-65% and an SSD drive not beyond 80% for maximum efficiency. That should also be a consideration when determining the size of storage.