out of disc space but not really

Ive been doing some live recordings. 30 - 40 min sets. 12 tracks or so. I save to an external hard drive (500G). A few times the recording has stopped giving an error about 'unable to save project. disc might be full' or something along those lines. However, I know that neither hard drive is full. Is there a buffer setting somewhere that I need to extend or something to keep it from doing this?