If it takes 4 hours to write to the drive/tape, 4 hours to transport it, and 4 hours to read it back in, that is 12 hours. If you are sending it over a wire, s

If it takes 4 hours to write to the drive/tape, 4 hours to transport it, and 4 hours to read it back in, that is 12 hours. If you are sending it over a wire, say the wire speed is 6 hours, the reading and writing are happening at the same time as the transmission. So total time is still 6 hours (you don't have to read the entire drive before beginning the transmission, and you write out to the remote side as it is being received).