On ubuntu servers trying to unmount a frozen nfs mount, I see “device is busy”.
When I try ‘df’ I see an Input / Output error for this partition. When I try ‘ls -la’ I see question marks ? where information should be.
To get it unmounted I used the first comment on this page;
umount -l /dev/sdwhatever
umount -l worked like a charm. Note that the original post with fuser cmd didn’t work for me.
After unmounting, I was able to remount it right away without any error.