Um, what sort of BSD are you using? When I do "man sed" on freebsd (and the bsd-based darwin on my mac), I see: 
  -i extension
      Edit files in-place, saving backups with the specified extension
+.
      If a zero-length extension is given, no backup will be saved.  I
+t
      is not recommended to give a zero-length extension when in-place
      editing files, as you risk corruption or partial content in situ
+-
      ations where disk space is exhausted, etc.

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In reply to Re^5: Strange Problem by graff
in thread Strange problem trying to clean garbage from start of mailbox file by capoeiraolly

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