Hi Asher_2,
Is your SD card is working properly? It might be possible, there is problem with your SD card. I would first suggest you to check your SD card by inserting it to computer or laptop. If it is supporting and still not able to transfer data then there is one option left. Just connect your phone to PC and copy all your data which you want to transfer to SD card and then connect your SD card to copy entire data to SD card.
Thanks!!