I owe you an apology Kovid.
I don't know how many times I've installed portable, has to be a several hundred times (more times than the standard install) and I can't recall ever seeing that message. I must have unconsciously ensured calibre was closed every time. I know I do it consciously when I install a new 64 bit version, so somehow that discipline must have leaked over into installing portable.
That I never made that mistake until yesterday will bother me for weeks
Again my apologies.
BR