Hi Guys,
using the sd image of a friendly helper I could restore my kobo h2o from the "flashing screen white and black and then shutting off error". I also restore the image on a 32 GB Micro SD and grew the partition via parted to the max. Initially the partition wasn't recognized correcly, but a kobo factory reset fixe the fat partition and filesystem. My Kobo H2O now has about 28GB free for Books and Dictionaries on the internal SD.
Regards
A big
for all the great people which helped me out...