Running out of space - what can I delete to create a new backup?

STATUS QUO:

I have found a filesize check here

which explains a bit more than I will explain here. Be aware that the syntax over there has changed a bit since at least regarding folder / file depth which is not “-d 1”.

du -h -d 1 /

Result is this, which I have manually formatted and resorted to make it more readable

1400M /
 569M /media
 483M /var
 313M /usr
  44M /config 
  13M /package
  11M /root
   7M /lib
   3M /bin
   2M /etc
   1M /sbin
   1M /run
  76K /tmp
  60K /data
  52K /command
  12K /home 
   4K /opt
   4K /mnt
   4K /srv
   4K /ssl
   4K /addons
   4K /share
   4K /backup
   4K /lib64
   0  /sys
   0  /proc
   0  /dev

Strange thing: if you add it up you will have a sum of just 2.847 MB but I have a 8 GB SD card and still just 0,6 GB free and I am not able to run a backup.
Yes, i have restarted the system 2 times fully.

What is wrong ?
And what about 1,4 GB of files in the root directory I can not see?