By default, ext4 reserves space that only root can use. Makes sense for system partitions. But for e.g. a storage drive I'd rather have that 5% to use for files.
See this askubuntu
sudo tune2fs -l /dev/... # list details
sudo tune2fs -m 0 /dev/... # set reserved to 0When making a new ext4
sudo mkfs.ext4 ... -m 0 /dev/...