When I used gimp to save a file to a SDXC UHS card, I got this message.
Read only file system.
The card was in a slot on my printer.
I used chown to make me owner with read/write permissions.
drwxrwxrwx 1 andy andy 131072 Dec 31 1969 3563-3531
3/13/19 Update
I can write to the SDXC UHS card when card is inserted into card
reader on my desktop.
Maybe the printer reader slot is designed to be read only?