Thanks everybody for your help and support.

At the moment I'm stuck with the partitioning part following the link
mentioned by tioubxancur.

I can follow the instructions on my N9, but the text is too small and
the wraparounds are annoying. So I tried to connect to the N9 via
ssh - works for "user" but not for "root" - can't login as root.
As "user" I don't have permissions.
So I will have to do some reading and trying.
Maybe exporting the MMC to my linux box is easier because I can
use gparted, but I have never done that.
Any suggestions ?

OT: I got my Jolla tablet today so I'm stuck between work and a honey pot