I ran into this last weekend with my trimslices and trying to use MMC cards in them. The mmc uses point contact 'brushes' to touch the card. Almost anything can make this flake out from corrosion, skin oil, temperature differences causing the brush not to contact carefully, bent wires etc. [The problem with mine turned out to be heat oriented.. if the system was cold it could read the card well, once it got warm it stopped seeing the card or would report other errors] I found I had to use a USB key in the end to boot the system as the connectors in that are much less prone to accidental damage.