Same thing happened to me when I started using my FP4. What brand is your SD?
Maybe you can check here ✏ List of compatible micro SD / SDXC cards for FP4 as a first hint, as this overview is only for the FP4.
You could also FP contact support (I did that a year ago) and ask for a hint which brand or model to use.
And the Tip of @Martin_Anderseck is the best to access your data: Use a card reader to pull the data from the card and - if you decide to get a new one - push the data to the replacement.