I had another look at this for you. I think its just C# mucking around. If you dont know C# then its kind of hard to explain. But basically :
I am loading the file, using it, then when im done im closing the file + It goes out of scope. EVENTUALLY the file will be released from memory of the application. C# has a built in memory cleaner(kinda) (Garbage collector), but the time it takes to clean up isnt pre defined. So it may be immediately, may be in a minute. In anycase, i think untill the memory is cleaned up, that the file will be locked in a state of "writing" and you wont be able to modify it.
I can fix this, i can immediately dispose of the memory using the dispose method of the object. I just didnt think it was that important (Infact i would be suprised if this object's dispose method does anything).
But i guess the quick fix for this is just not to save the file quickly one after the other type of thing. Otherwise it may just lock the file.