When your phone is full but you don't want to lose your files and you are too busy to move them to another place, you probably think about that microSD card that you bought some years ago. But, here comes the problem: your device doesn't want to accept that card.
Your microSD probably has errors and your phone is not recognizing that card. Try connecting a microSD card reader to your PC and format the card at least 2 times. Restart your mobile and insert the SD card again.