The file system is read only on the affected machine, fortunately I remembered the 'tee' command and had a USB stick.
So in case of others having similar problems...
I noticed messages about 'sdf' on the console, when I inserted the USB stick, so I ran
mount /dev/sdf /media/
and saved the output and saw it on the screen by doing things like
cat /proc/mdstat | tee /media/gcf-mdstat
note that 'tee' does not send error messages in the file