My hard disk suddenly full

I had a similar problem and it turned out a log file was getting massive.

Change to your Home directory:

cd /home/*yourusername*

ls -al 

check to see if anything is way too big. My .xsession-errors file was 430GB!

So I deleted the darn thing:

sudo rm .xsession-errors 

There are several tools that can display your disk usage graphically, for example filelight. Without futher investigation, it is nearly impossible to say what causes your disk to get full. If you get that additional info, you should post that in your question to get more precise answers.

For me, the downloaded packages (.deb files) are often a source of high disk usage and you can clear that using the command apt-get clean.

And like the other answers state, I can't see why your disk shouldn't be mounted correctly.