I think I found something. If you look up the FileMode enumeration, OpenOrCreate does partially what you want except it doesn't seek to the end of the file. Meaning each new chunk of data would overwrite the last. Not sure if that's whats really going on since you said you get more data on the server side then you sent from the client. I believe what you want there is FileMode.Append, which will create the file if it doesn't exist, or open it and start the writing at the end.