Thanks for the reply.
I’ve now tried both your suggestions with odd results.
Mapping the share to a drive letter did allow me to successfully copy a file onto the WDTV’s local USB storage - but only once.
The next time I tried (much later after a computer reboot) it failed again, and hasn’t worked since.
Oddly, exactly the same effect with using another PC.
I dragged out my laptop and it was able to copy a file to the WDTV, but again only once.
The second time I tried I got the usual error.
I’ve now discovered that I can copy files from the network using the WDTVs own copy utility. So far that’s working consistently.
So it must be a Windows thing, right?
Also as a Community newbie, I see now that I’ve posted this query to the wrong area (Live Streaming).
Would it be OK to repost my query to the Networking area, or would that be regarded as spamming?