APPEND : String = "append"
[static] Used for a file to be opened in write mode, with all written data appended to the end of the file.
https://help.adobe.com/en_US/FlashPl.../FileMode.html
You should make sure what you want to use. Append probably just keeps adding to the file until it gets too big. I figure that the file you try opening ends up being too big.