Like I said before, you have to make sure you use the right Write method for the type of data you're dealing with, again, a GUID is NOT a regular integer.. you even defined it as a UInt64. So the next logical step would be to use the correct Write method.
Without too much spoonfeeding, take a look at the links posted here, and try to understand your code. Like, can you explain why you use UInt64 to hold a GUID? Or why you use UInt64 to hold the address you want to write to?