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As you say, something in Windows is preventing...
Valid troubleshooting will be write to a different usb drive in that port, write to that drive in a different port, test with another pc....
I would leave calibre out of it until you have worked thru the above permutations.and, obviously, check or suspend AV settings.
sone AV may be blocking potentially executable file types so txt is allowed, epub being a type of zip is not...
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