Post-Maverick upgrade, now my firewire MyBook does not mount

WD External Drives External Drives for Mac
2

Mavericks caution, upgrade your WD Drive Manager to the latest version available. 

http://community.wd.com/t5/External-Drives-for-Mac/External-Drives-for-Mac-Experiencing-Data-Loss-with-Maverick-OS/td-p/613775

I hope that is only formatting issue … and that is not an old version of WD drive manager have deleted your data :cry:

in case of problems on mac OS format (when Disk Utility does not work properly), can be solved using DiskWarrior.  http://www.alsoft.com/diskwarrior/