We tried a 32GB card first, the update seamed to run fine. To finalize the update file is deleted from the sd-card. This all happened, then the device was stuck in firmware-update mode (undocumented state).
Tried it again with a 4GB sd-card, update worked fine.
We assume the update file was spread beyond the 8GB boundary
I'm scared to try an unauthorized card in my Nintendo Switch because I could lose my Zelda save file.