Western Digital My Book Thunderbolt Duo will not mount.

"Do you have access to a second mac? I would give that a try.

Depending on the model both thunderbolt ports are using the same controller logic so if one goes its possible the second has gone as well.

How do you have your equipment powered? Are you using a surge suppressor or a UPS and are pall of the devices sharing the same surge suppressor or UPS? Often I hear burnt ports due to power or lightning damage. I strongly recommend installing a whole house surge suppressor (in your fuse panel) and use a second one for the equipment or better yet use a UPS.

If you get power disruptions or lightning storms then a UPS is a must."