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.