 Tracks could thus start anywhere, but the copied track had to have this "write splice", which always caused some bits to be lost or duplicated due to speed variations, roughly in the same (unused for payload data) place as the original, or it would not work.Secondly, Apple used special "self-sync" bytes to achieve agreement between drive controller and computer about where any byte ended and the next one started on the disk.These bytes were written as normal data bytes followed by a slightly longer than normal pause, which was notoriously unreliable to detect on read-back; still, you had to get the self-sync bytes roughly right as without them being present in the right places, the copy would not work, and with them present in too many places, the track would not fit on the destination disk.