OK, so I went through my inventory of four USB Micro cables and two USB hubs. The fault was in the cable I used initially, in fact only one of the four cables I tried worked as it should. Cheap USB cables - you get what you pay for I guess. At least, Calibre, my computer and my e-reader are able to communicate again.
Anyway, thanks for the help, @theducks and @BNSB
