How to continuously gather (and save as several files) a large amount of data from NI usb device with matlab?

Hello, I have a question about reading data from a NI USB device with matlab (and the data aqcuisition toolbox). I know that it is possible to communicate with this device, I have succesfully done so, but I wondered if it is possible to gather continuous information and save the data in sections. This sounds simple, but the problem is the amount of data that will be gathered and the sampling rate. I want to measure about one hour with at least twenty channels with a rate of 20kHz at each channel. Is it possible to save a number of samples as, for example an ASCII file, close this file and then save the next number of samples and clear the buffer from the samples that have been read. I do not need to process the data right away so I do not want to save it to the workspace of Matlab. Does anyone know if this is possible with Matlab, and if so, how to do this? What steps would I need to take?