When you plug in a "eXtreme Capacity" Flash memory card (including SDXC or micro SDXC) into a either an external USB Card Reader or a card reader built into your computer, that reader must be compatible with the SDXC standard. SDXC cards are not compatible with older card readers. Inserting an SDXC card into a non-XC compatible reader could have unpredictable results from simply being not recognized at all to potential corruption of critical file structure data that would make the card inaccessible.
You might see an error that says "Please insert a disk into drive x:" like this:
Or worse, the error might say the drive is inaccessible or there was an I/O error.
1. Make sure you are using card reader compatible with SDXC.
2. If the card still works in your digital camera, try connect the camera to your computer with the camera's USB cable and access the card through the camera's card reader.
3. If you think you might want or ever need to recover the files from that card, do NOT format or attempt to put more data onto the card.