Well there's interesting.............

All of the discs report absolutely no errors, however only the discs burned on a later DVD burner (2007 vintage) and using Vista OS actually work, and I for one have no explanation as to why. It's the same files, same versions of either Imgburn or DVD Decrypter and the same DVD media types - just burned on a different machine. The file sizes all match too, but for some reason the Mk4 will not read discs burned in the older (2003) vintage burner - even though the PC can read all the data perfectly.

A very frustrating 24 hours - and no obvious reason why one works and not another. Thanks for all the hints and tips though - it certainly pointed me at the right things, but the burner & OS change was not what I expected to resolve the 'challenges'!!

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Me:
Current: 2011 E71 40D Vermilion Red/Beige Nappa

Wifey:
Current: 2012 R80 Countryman SDX Green/Cream