Hi Chris,
Following on from my comment to Bradygaster I discovered that the issue I was having was related to calling File.Delete(filePath). At this point the board became unresponsive and I had to wipe the application from the board (Netduino Plus 2).
The code had worked for some time without an issues and then started having these freezes (reproducible every time). In the end I had to reformat the SD card and since then it seems to have been working ok.
My only guess is that buried down in the NETMF code it got stuck trying to access the SD filesystem and couldn't break out of either a resource contention or attempted to access the required resources without any sort of timeout.
Thanks,
Jason.