Hard to say without seeing your code etc... and i'm not a PI expert, but something that springs to mind - are you releasing the text file after you have written it? if you are using StreamWriter to write to your text file make sure the object is disposed once you have finished writing.

Hey guys! I'm looking to port over some apps that I have developed for Windows, Apple and iPhone over to the Pi. My plan was to use Mono on the Pi and then use a modified version of my C# Windows software. Is this a good idea? My app has a graphically rich user interface, which I create the long way...