Thanks very much to all who replied. I found out why, AND how to fix.
WHY : I was running "format" in a Command-Tool Window. I now remember - I did
  click on another window to bring it in front, and not long after that, format and
  the command tool and my disk executed a suicide pact together. In case I am
  not the only naive fool : 
  **** DON'T RUN FORMAT IN A COMMAND-TOOL WINDOW : USE SHELL TOOL ****
  (personally, next time I will exit windows, go to single user mode, and wear an
  anti-static wrist strap, before running format)
HOW : The following format.dat for this drive worked fine for me, allowing me to
  re-label the disk :
disk_type = "SUN1.05" \
        : ctlr = MD21 : fmt_time = 4 \
        : ncyl = 2036 : acyl = 2 : pcyl = 2038 : nhead = 14 : nsect = 72 \
        : rpm = 5400 : bpt = 43200
(BTW Sun were also very helpful - I actually got this format.dat from them)
*** Thanks again *** 
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