Ok, I've solved it. It was necessary to install the Nokia PC Suite (which is a really sloppy bit of kit, if you ask me).

On connecting with the suite, disconnecting, and reconnecting, Windoze went though a bit of song and dance with the task bar balloons, and then said everything was A-OK. And the 1.2GB file that I just transferred in about 20 minutes, I transferred again in under two minutes.

I'm surprised something like this doesn't get more talk around here. Unless everybody is installing the Nokia suite as a matter of course? I didn't even know where to get the suite - I had to Google for it - and I wasn't even looking for it, I was looking for USB2 drivers, but all I found of interest was the suite, so I decided to give it a try. Glad I did!