database or disk is full : the wrong disk?

I just tried to add an object on my django app.

But I got a "database or disk is full error" --- and indeed I have a full disk.

But the full disk is the one I mounted to my /, and the one containing my django app is the one mounted to my /home and this one isn't full.

So why do I get this error? How is my root used for a django app (and database management)?