Probably, sometimes :-). Without information on how the file system, device driver and device firmware allocate space at that particular point in time you have no way of knowing. sysread is too far removed from the actual disk operation to make a guarantee on how one impacts the other. In general this may work, however, there are some cases where it definitely won't (e.g. Flash drive, different sized data, remapped bad sectors/clusters) and AFAIK the behaviour in other cases is undefined.