Since I last cloned my HDD, the 'continuing' system has developed the problem in the title/subject. Luckily the clone was made before that 'corruption', so MP4s in it still show thumbnails. Much discussion 'on the web' about this topic suggests the icon cache may be corrupt, so I made the main HDD a slave to the clone, and cleaned out the icon cache, and refilled it with a copy of the clone's cache. NO improvement ! What could have caused the corruption, and more importantly, how can I fix it again ?