of pack in buffered mode, then switching to unbuffered mode to copy the
tail, actually broke pack completely on Windows:  we didn't close the
buffered file handle before opening the unbuffered one, and self.gc held
on to the still-open former handle.  This prevented the caller from
renaming Data.fs to Data.fs.old (a handle on Data.fs was still open).
The cure is simply to close a handle when we stop using it.