Galaxy A8/A8+: How do I format the memory card ?
A memory card formatted on a computer may not be compatible with the device. Format the memory card on the device.
Before formatting the memory card, remember to make backup copies of all important data stored in the memory card. The manufacturer’s warranty does not cover loss of data resulting from user actions.
On the Home screen, Tap Apps → Settings → Device maintenance → Storage → More option → Storage settings → SD card → Format.