Tom Benstead wrote:
I would have thought
Ethereal would have used the contents of cfile.pd to
create the new capture file but this does not seem to
be the case.
No, it's not, and it never was intended to be.
cfile.pd is a *temporary buffer* to hold the *current* packet; there 
isn't even any guarantee that any changes made to that buffer will 
persist across clicking on a different packet and clicking back to the 
"changed" packet.  The permanent home of packet data is the capture file 
*itself*.  The only way to make packet changes persistent would be to 
modify the capture file - and that'd be very difficult if you were 
changing the size of the packet or inserting a new packet.
Ethereal wasn't designed to be a capture file editor; there would 
probably have to be very substantial changes made to it in order to make 
it one.  (For one thing, as packet dissection is stateful, i.e. the way 
a packet is dissected could depend on the preceding packets in the 
capture, changing a packet could require that all the packets after it 
be re-dissected, and that could involve discarding some state and 
regenerating it, so the simplest and safest way to do the re-dissection 
would be to re-dissect the entire capture.)