Disk space disappearing

Suddenly I started to get low disk space warnings on the EXT4 partition mounted as / with no apparent reason.
I assumed it was some problem with EXT4 as I was sure nothing could fill the 10Gb I had free the previous day...
I started by using Disk Usage Analyzer trying to find out where all the space was going but found nothing... the space seemed to just disappear with no content. Using fsck to check the partition for errors was not helpful also.
Then I found this wonderful guide about 'Disk Space Problems & Solutions' which led me to find out the problem was with 'Simple Backup' trying to make the backup to the wrong place.
This happened because the backup started without the external drive mounted and so it created the path where the drive was supposed to be mounted and created the backup there. The next time the drive was mounted the mount point was taken by the backup so it mounted on a different location and the subsequent backups were stored on the laptop drive, eating GBs of drive space every day.
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