By default ext4 reserves 5% of your disk for the root user. That made sense in 1999, but that’s insane now. You can run this command (as root) to reduce reserved blocks to around 4GB. On a 16TB ext4 file-system that will free up 795GB (since the default of 5% reserved = 800GB reserved for root!).
tune2fs -r 1000000 /dev/<disk-device>
This instantly frees up the space. You don’t need to unmount or remount or restart or anything.
But first use “lsblk” and “df” commands to identify the ext4 file-systems to tune.
And be careful! Running any command directly against any
/dev/<disk-device> file as root is always dangerous. Practice on a computer/drive that’s not important first.