RPi is reporting a full disk, but counting the total folder disk size doesn't add up

[/RaspberryPi]

I have a Pi 4 with a 32GB SD card loaded as the root disk space. Recently I've been getting disk space full errors and decided to check with df to see that the root disk is indeed completely full.
pi@rpi:/ $ df -h Filesystem Size Used Avail Use% Mounted on /dev/root 28G 27G 0 100% / devtmpfs 775M 0 775M 0% /dev tmpfs 936M 0 936M 0% /dev/shm tmpfs 936M 98M 839M 11% /run tmpfs 5.0M 4.0K 5.0M 1% /run/lock tmpfs 936M 0 936M 0% /sys/fs/cgroup /dev/sda1 253M 50M 203M 20% /boot /dev/sdb1 1.8T 821G 919G 48% /mnt/disk1-toshiba-2tb tmpfs 188M 0 188M 0% /run/user/1000 So in an attempt to free up some space I decided to check my disk usage with ncdu and du but when counting the used space per folder it doesn't add up to the 27G used:
820.6 GiB [######### ] /mnt 4.4 GiB [ ] /var 2.1 GiB [ ] /home 2.0 GiB [ ] /usr 100.3 MiB [ ] /opt 97.4 MiB [ ] /run 49.3 MiB [ ] /boot 6.9 MiB [ ] /etc 3.0 MiB [ ] /root 40.0 KiB [ ] /tmp e 16.0 KiB [ ] /lost+found e 4.0 KiB [ ] /srv e 4.0 KiB [ ] /media 0.0 B [ ] /sys . 0.0 B [ ] /proc 0.0 B [ ] /dev @ 0.0 B [ ] sbin @ 0.0 B [ ] lib @ 0.0 B [ ] bin I'm counting the largest dirs (4,4+2,1+2) which would only accumulate to 8,5 GB used. Why would my disk space be full and report 27G used even when the total in folders is only 8.5GB?
There are no other folders in /mnt aside from the external HDD disk mount from /dev/sdb1

submitted by /u/thibmaek
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