Wikipedia has documented the known issue with users of Western Digital external hard drives who upgrade to the OSX Mavericks. Here’s the link.
To summarise, issues became apparent when some users of WD products and especially WD disk management software upgraded to Mavericks. The most common issue was hard drive data loss &/or data corruption. WD were quick to withdraw the problem causing software from download and issued new versions that did not cause data loss issues with Mavericks, but for some users the damage had already been done and it was often not possible to reverse this either.
If you’ve been affected with this problem you may like to try talking to a decent data recovery company, there are reviews of them on Google. My advice would be to avoid any DIY or free software methods as they could easily make matters worse. If there is any software issued on the WD site that addresses the data loss problem you should be ok using that to help you.