Hi Doug,
This is FAQ number 3. Use the "-a -G1" options and you will see where your serial number information was written. You can also use the "-v3" option when writing to see what is happening.
Unfortunately, the Nikon serial number for the D200 is not writable (see the Notes for SerialNumber in the
Nikon Tags documentation). This is because it is used as a key for the decryption of other information. So you are instead writing XMP-aux:SerialNumber, but you need to use the "-a" option to see this duplicate information when you try to read it back again.
- Phil