Thanks for the clear explanation and the provided link about unmountable disks!
Step 4 says: The section following that is the one you want, titled Check Filesystem Status. There is a box with the 2 words Not available in it. This is the command output box, where the progress and results of the command will be displayed. Below that is the Check button that starts the test or repair, followed by the options box where you can type in options for the test/repair command.
I don't the words "Not Available", but "Unmountable: No file system " behind "File system status". Also the file system is set to AUTO and not XFS which I would expect, but maybe that is as expected. No CHECK button whatsoever to play with. Am I looking in the wrong place?