Check your anti-virus to see if you have a setting to not allow external media. I went through all of the steps, typing in the Terminal line, running Disk Utility First Aid, trying to make the disk mount... Nothing worked. Finally made an appointment with Apple Care, the technician did an diagnostic, and they figured out it was the virus protection they sold me (ESET) that was blocking it. A simple checkbox. Argh! But fixed now.