Actually if you run in terminal command like "ps ax | grep fsck" in the Terminal, you will see that filesystem check is already running (and checking your device). And this is why clicking on "mount" or "first aid" will return error, because it on repair right now. And when it finish disk will be automatically mounted and will be working again. However it can take a lot of time and for my 2 TB WD disk the time was about 2 hours. So be patient and let the system check your drive fully.
virtustilus
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