Just for my own records: if you ever face problems with renitent USB flash drives, here is the answer: the command line. See the answer in this post.
https://discussions.apple.com/thread/8132218
Author: Oliver Busse
http://about.me/oliverbusse
This software is licensed under the Apache License 2.0
Code snippets or any other console output - if not otherwise proclaimed - licensed under WTFPL – Do What the Fuck You Want to Public License
Made with love and IBM XPages, using Twitter Bootstrap 3
This website uses the XPages Extension Library, Bootstrap4XPages Plugin and the OpenNTF Domino API