I can not use a custom shutdown script. The usb drive is still mounted under /var/log and /storage with open files, e.g. systemd logging daemons and also the shutdown script itself which is in /storage.
Any other Idea?
I still think it is a bug in the systemd shutdown process. Maybe its not forced to first umount and then switchoff power of usb. Maybe its allowed to do in parallel. Systemd tries to do as much as possible in parallel.
Problem is I still did not find the script/process which powers off the usb. Then I could add a dependency to first umount