I have a similar problem with MY Book Essentials USB 2. I have contacted support, but until they respond I go into the Task Manager and terminate the WD Smartware.exe task. I can then remove the hardware with no problem.
Neither did I. Apparently it loads when the drive is plugged into the USB port. Check your Processes in Task Manager to see if WD SmartWare.exe is running.
I had similar problem (unable to safely remove …), spent hours to solve that - turning off antivir did not help, ProcessExplorer showed no process using the external drive …
Finally I ended up with uninstalling one piece of software after another, disabling uncriticall services etc … what helped me was to remove/uninstall virtuall PDF Printer that came with Software 602 Form Filler. I don’t know what this piece of SW had to do with the external disk, but now everything works fine.