…reformat the drive from Disk Utitily with Mac OS X Extended Journslised. Forget about WD’s own software and firmwire. And then start Time Machine again and it’ll work just fine.
You’ll need to backup all the files in the drive first thouhgh before reformtting.
I discovered this when I bought a LaCie drive to replace my WD MyBook Studio because the infamous firmwire 3.4.4.2 would not work and even if it did, it would have destroyed the drive anyway as mentioned seceral times in other threads. So I had no other option but to buy a new LaCie drive to back up with Time Machine. Like with WD, Lacie came with their own bundled software and reformatted the drive throuhgh their set-up wizard. But still Time Machine would not work in a similar way as the WD. So I decided to reformat the drive using Disk Utitily as mentioned above and, bingo, Time Machine worked and was able to back up all my files into it.
Then it hit me. What if I could do the same with the WD drive? So I reformatted it using Disk Utitily and then used Time Machine and guess what? It worked perfectly fine and was able to back up all the files into it without any hitches.
I reckon Mavericks’ Time Machine wants a totally clean external drive without any obstacles such as the bundled software to make it work.
Problem solves. 
