Are you by any chance using Acronis backup on your PC? I had exactly your problem and traced it to the Acronis USB drivers not supporting all USB devices properly. A work round is to go into Device Manager and disable the problem devices then immediately re-enable them whereupon they appear in Windows Explorer and Calibre sees the PRS-T1.
This worked for me and its probably worth a try even if you aren't using Acronis.
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