Good news,
Something is missing in the documentation:
When we copy the new Navio image to the SD card, the root volume stay with a low size (2.9 Go for the latest) even if the SD Card is larger (8Go)
Check the partition size with:
lsblk
There is a command line to enlarge the partition:
sudo raspi-config --expand-rootfs
sudo reboot
then check the size:
lsblk
Marc