Whenever I try to inject a tune setup from forza 4 to forza horizon and go to save I get that error. And yes, I have diamond. I bought a month last night so I am hoping to be able to get this fixed so I can actually use it for what I bought it for. 
It also happens when I change the hex of the file when I press save.
I am honestly not sure @sNaKe, I haven’t used XP in over 3 years now.
I DO KNOW that, with winXP being officially “obsolete” and no longer supported by M$;
if it is a .Net, or any other win based issue there really no guaranteed definite fix.
@XENO4x86 have you used, or can you use Your HD for any other game titles? I ask because that will rule out (or point to) your system as the problem.
ALSO–IDK what process you are using to put FM4 data into FH, but there are certain things You have to do so that the setup stays (of course it never will be 100% due to different dev/game engine) as built in FM4.
Without trying to come off any type of negative…but when You have Forza, Hex, and Save all in the same subject, You are pretty much saying that You have come to play in MY sandbox.
LMAO been doing it too damn long and it is THE ONLY REASON I ever bought Diamond or kept up on it. I am guessing that would be why sNaKe tagged Me in on this aside from having mentioned a lot of XP experience.