CF Card failure - a bit different

Memory Cards Mobile memory, professional cards and all other st
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I endured a very painful experience for my Ultra 8G CF cards in the past few months too. As a heavy user of my Canon 7D, I owned more than 10 pcs of the Sandisk CF cards including Extreme and Ultra series. Each shot activity I will use around 7 pcs or more of them usually. For the concerns to lose all the photos for any single physical failure in a card, I tried not to use any memory cards with size larger than 8GB.  My photo-taking operation is very simple, only one camera but two Lens; no CF cards change unless its space used up; and only one PC with windows 7 and an USB 3.0 card reader for photos download, that all. The photo taking is always no issues at all, but the download is quite unreliable. More than five times in 2013, one of the cards among the 7 pcs which I used cannot be detected or read from my PC after downloading around one-thirds of photos. However the remaining CF cards are readable even after the faulty card case.  This faulty card is end-up with a return for new replacement because I cannot reformat it or work with my camera afterward. Just wonder what’s the root cause of the issues and how can I do if that’s my fault.