 The data read ahead of request during this wait is unrequested but free, so typically saved in the disk buffer in case it is requested later.Similarly, data can be read for free behind the requested one if the head can stay on track because there is no other read to execute or the next actuating can start later and still complete in time.If several requested reads are on the same track (or close by on a spiral track), most unrequested data between them will be both read ahead and behind.