There are a few things that can cause this, so I can't say it is a specific one. However, to fix this issue I would suggest two things.
1. Remove all photos and other files off the card onto the computer.
2. Put the SD card back into the tablet and format it. Then leave it in the tablet.
It should work from this point on, as long as there is nothing wrong with the card or the tablet.
In the future, you should not take the SD card out of one device to use or access in another one. This causes problems (like this one) and can also damage the card, not to mention damaging the device you are putting it in/taking it out of. They aren't really meant to be moved from device to device. When you want to transfer files from the tablets SD card to say the computer, you should connect the tablet to the computer via USB cord and access it that way. You don't need any special software. Just open a folder on the computer and access the device. It should then give you the option to access the devices memory or the sd card.
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