Person

I used to get this whenever an external HD lost connection while mounted. What worked for me: Make sure you are NOT trying to mount the volume as read-only, since that prevents the operating system from doing whatever it needs to do to fix the file system after a hot disconnect, thus making the disk unmountable. Mounting as read/write fixed it for me – after that you may continue mounting it as read-only.

I used to get this whenever an external HD lost connection while mounted. What worked for me: Make sure you are NOT trying to mount the volume as read-only, since that prevents the operating system from doing whatever it needs to do to fix the file system after a hot disconnect, thus making the disk unmountable.