xPC file scope which overwrites except when specified

I would like to create a file scope which is continually saving data to a single file. This file will be overwritten except if a signal is received to keep that segment. I was thinking about using a signal-triggered file scope with pre-trigger sampling. However, I need ~30 minutes of samples. Can pre-trigger sampling handle this? Also, what happens if the the target has not run long enough to fill the pre-trigger sample buffer?