So finally .. i managed it...

I downloaded following iso https://sourceforge.net/projects/gparted/

With AcetoneISO downloaded from Softwareshop under Linux Mint 18.3 on MacMini i decompressed the iso to the usb stick.

The i take my USB Hub and bootet from the Stick.
I used default settings.

After a while the Debian Desktop was shown.
So i expanded the Sailfish Partition to the maximum.
Now i had 24,7 GB free.