I explained a little bit here about why I think you cannot expect the "To File" block to write to a file immediately:
"I think the reason that To Workspace blocks don't write the variable immediately it because it is very inefficient for To Workspace/File blocks to write the workspace/disk at every time-step, because these are slow processes than will slow down simulation. My guess is that they maintain a buffer that they write to at every time-step - the data is probably written to the final destination only when the buffer is full."
