Maybe it helps someone - I messed up my microSD with a former btrfs format and @chemists android mount - something went wrong after an update and the card became unreadable and unfortunately unformatable - tried it with OS X, Linux and Windows as well as with several phones - nothing
Today I found my old N8 in the drawer and remembered the workaround for the WP7 sd cards and thought to give it a try and - success :-) - the N8 told me that the sd is password secured in first place but in the file manager I was able to choose format and - magic - it's back to live

What have we learned today - keep your old hardware, it can do things where modern devices struggle ;-)

Btw Symbian is still annoying from the point of user experience;-)