External drive share failing

This may be more of a Windows 7 problem, but thought I’d ask anyway…  I have a 1TB WD Elements (FAT32) attached to my ATV1; all works fine via nitoTV, XBMC etc.  I’ve also set up mapped drives on my Windows 7 (x64) machine to access the three folders I have on the drive - TV Shows, Kids and Movies.  Here’s the problem:

I have an automated setup (using uAL, uTorrent, Sickbeard, Handbrake WatchFolder, Handbrake Batch Encoder and Belvedere) to locate, download and convert video to the appropriate formats…  However it fails at the last step - copying to the appropriate share; saying the the network resource cannot be found.

I can see the ATV in my network, and can browse the folders - but copying anything across fails (I have full admin rights to everything, BTW).

Has anyone come across anything like this before?  More usefully, has anyone encountered this and fixed it?!