Sony PCM-800 Operation Manual page 49

If you attempt to record additional material to augment the original
and wish to format the tape from that intermediate point, be sure to
sufficiently move back the tape beforehand to prevent an
unformatted section being left in between.
When the tape runs from the previously formatted part into the
newly formatted part, there could be discontinuities of ABS time
data. The transition could cause erratic synchronization if it happens
when synchronizing. If audio data is present over the transition
zone, noise could be heard. The best is format a tape from start to
end without interruption.
Chapter 5 Operation
5:3
Chapt HH