How long have those 900+ photos been on that card? Do you do any in-camera deleting? I ask because sometimes the card data can get fragmented. I haven't experienced this but a friend that owns a D90 did, he would fill the card, delete inconsequential photos, and shoot some more. He rarely offloaded them to the computer, as such he was just shooting pictures for fun but found out the hard way when his D90 would write to the card anymore until he manually copied the photos off and reformatted it.

Before every shoot I start with a blank memory card, I format it in camera and don't delete anything until the photos are offloaded into my computer.

Have you checked your camera/lens contacts to make sure they're clean? Maybe something isn't making 100% connection.

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