Oh, that's strange, but we can figure this out. Does your card fit in snugly? Try taking it out and removing, then reboot. Does it say unavailable or any errors there? Do you have anything saved on the SD card?
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Either does mine as of yesterday, I blame Verizon for this, as I updated my backup to Verizon Cloud, then everything disappeared from my card and now my device can not find it although it is a brand new Micro SD Card. Be very careful, the Cloud eats all your data! What I would like to know is if they are doing this on purpose. So we will have to upgrade our device. I've been reading for a solution all day today on Android forums and there are a surprising amount of people who are having the exact same problem in the past week! If I do not have mobile data access since I live in Tijuana and can't afford it all the time, how can I listen to all my music? $100's of $$'s of music gone from my device. Verizon Cloud does not tell you ahead of time that they are going to erase your SD card contents. I wish I had never updated to Verizon Cloud!