Totally agree Kovid,
However IMO your earlier post implied that an autosave written for the purpose of recoverability couldn't be done atomically. But I don't want to get into a semantic argument about it.
I was minded to move the thread back to Sigil, following post 10 from Ripplinger. There's a good chance the EPUB is in %LOCALAPPDATA%\Temp\sigil\scratchpad
Moved back to Sigil - OP's problem has nothing to do with Calibre
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