Long story short, I woke up this morning and my 4TB WD USB 3.0 HDD which has been hooked up to Time Machine was mounted on my desktop and totally empty. It appears to be fully functional according to disk utility. I have not idea how this happened.
I am running a rMPB 15" 2012 (max specs) with Mavericks. It is connected to a Dell U2713hm Monitor with USB 3.0 Hub. It is also connected to three external USB HDDs. I have two of those WD HDD's both running Time Machine backups for my computer (one after the other back and forth). My 4TB WD HDD is plugged into the USB 3.0 hub on my Dell Monitor.
So I partitioned my 4TB WD HDD into five partitions so I could run Time Machine on it with four computers (I was planning to plug and unplug them to the USB port). One of those partitions was doing Time Machine backups for my rMBP on a regular basis (it was constantly plugged in to it).
Disk Utility says that the 4TB HDD is running fine. But this HDD does make slightly more noise than usual. I don't know whether or not that noise means that the disk arm is failing or not. In any case, I woke up this morning and my 5 partition backup HDD was gone and in it's place was a one partition Mac OS Extended (Jounraled) formatted empty 4TB HDD named "My Book". Does anyone know what happened to me? Is my HDD on it's way out? Weird...
Since my drive in question was just a time machine backup I simply reformatted it and did a clean install of Mavericks.
It is important that you make sure the WD Utility software is no longer on your Mac or you may see this problem recur -- not just with your WD brand drive but with any other external hard drive you might connect to your Mac.
Running the WD uninstaller mentioned above should do this, but if you want to check that it did my post today in Mavericks corrupts external hard drive (on page 43) about the older manual method may help with that.