linux - du vs. df difference

I have a fileserver where df reports 94% of / full. But according to du, much less is used:

# df -h /
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda3             270G  240G   17G  94% /

# du -hxs /
124G    /

I read that open but deleted files could be responsible for it but a reboot did not fix this.

This is Linux, ext3.

regards

Answer

Ok, found it.

I had a old backup on /mnt/Backup in the same filesystem and then an external drive mounted in that place. So du didn't see the files. So cleaning up this gave me back my disk space.

It probably happened this way: the external drive once was unmounted while the daily backup script run.