Thanks for the suggestions.
USB stick worked for me. Takes quite some time to load, but better than plugging in the Etrex.
I tried creating folders on the HD with the map data therein, but couldn't get BaseCamp to recognise them as a map file, no matter where I located them. And I'm not clever enough to understand how to map a folder so that it appears to be another PC disk like the Mac users on the CTC forum linked
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Bryn