Ok so first hit Command+Space this will pull up Spotlight. Then search and open Disk Utility. Now open Spotlight again (Command+Space) and search for Terminal and open that. Now that you have both open I suggest in Terminal type the command he listed above sudo pkill -f fsc BUT DO NOT HIT ENTER YET!!! Now in Disk Utility click on the drive that is not working along the left hand side and then along the top of the window click on mount. At this point you will not see anything happening that's ok, it's working in the background. Switch over to Terminal and now hit enter for that command you typed in earlier. This will kill the stuck process that you just ran in the background which is the issue. You will get two error pop ups, acknowledge through them. At this point you can close out Terminal and on Disk Utility along the top you can click First Aid and run that. It will take a while depending on how large your drive is and what not. Let it run and you should be fine. Let me know if you have any further questions.
