I install Debian on a 16gb SD card, but I can only use 8gb, how can I fix it?
Check out this tutorial, works regardless of distribution. It’s best to perform it on standalone linux/osx machine, not the Banana Pi that uses this SDCard.
https://elinux.org/RPi_Resize_Flash_Partitions#Manually_resizing_the_SD_card_on_Linux