Problems Exporting Aperture Photos to MyBookLive Duo

Yes I have exported to other locations (external WD HDDs) but never had the problem on any of those. It seems intermittant on the Duo - transferred a few photo files to-day, including a 3.8 Gb movie file, without a problem. Frustrating having to check ‘Duo’ files by re-importing them to prove their existence is ‘robust’.