Mavericks is not compatible with the software that came bundled with some Western Digital storage devices, specifically the apps named: WD Drive Manager, WD RAID Manager, and WD SmartWare. This software must be UNINSTALLED prior to upgrading to Mavericks, or there is a high chance that connected WD drives will experience complete data loss. This has been reported by hundreds of WD MyBook owners, and numerous owners of other WD storage devices.

Western Digital has acknowledged this issue, and is working with Apple on a fix. They have posted a description of the problem and an uninstaller for their software here:

Western Digital has further stated “If you have already upgraded to Mavericks and are experiencing difficulty in accessing your external hard drive, please do not save anything to the drive, disconnect the drive from your computer, and contact Western Digital Customer Service”.