What firmware version is on the device? One of the recent versions, either 4.3.x or 4.4.x, would ignore the SD card if it didn't have a label. This was fixed in the next firmware release, but if you don't want to upgrade, labelling the card solved it.
If that isn't the case, what happens when you insert the card? Does the screen flash? Is the card seen by the PC when connected?
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