...for a small percentage of customers that have the WD Drive Manager, WD Raid Manager and/or WD SmartWare software applications installed on their Mac http://community.wd.com/t5/News-Announcements/External-Drives-for-Mac-Experiencing-Data-Loss-with-Maverick-OS/td-p/613777
External Drives for Mac Experiencing Data Loss with Maverick OS -- UPDATED FOR NOVEMBER 6, 2013
--- Updated November 6, 2013 ---
On October 30th, 2013 Western Digital informed registered customers of affected products via E-mail regarding reports of Western Digital and other external HDD products experiencing data loss when updating to OS X Mavericks (10.9). Our investigation to date has found that for a small percentage of customers that have the WD Drive Manager, WD Raid Manager and/or WD SmartWare software applications installed on their Mac, there can be cases of a repartition and reformat of their Direct Attached Storage (DAS) devices without customer acknowledgement which can result in data loss.
WD has been tracking this issue closely through our WD Forum and through our Technical Support hotline and the occurrence rate of this event has been very low. A specific set of conditions and timing sequences between the OS and the WD software utilities has to occur to cause this issue. Should this event occur, the data on the product can likely be recovered with a third party software utility if the customer stops using the device immediately after the OS X Mavericks (10.9) upgrade. WD will be issuing updated versions of these software applications that resolve this issue.
WD strongly urges our customers to uninstall these software applications before updating to OS X Mavericks (10.9), or delay upgrading until we provide an update to the applications. If you have already upgraded to Mavericks, WD recommends that you remove these applications and restart your computer. 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 at http://support.wdc.com/contact/.