I don't know if this will help anyone, as it has been about 7 months since the first post, but I've been fiddling with a new WD external 3TB drive and have had similar problems.
One thing to check is when you are in the disk management screen, the box on the left that tells you the disk number, size, and type of drive has a menu if you right click it. If it has an option for converting to a GPT disk, then it is probably formatted as an MBR disk, which won't allow 3TB partitions due to the sector size limitations. What you would want to do is select the option to convert it to a GPT disk (Make sure you have all your data backed up as you will have to reformat the drive and lose everything) and create a new single 3TB partition.
First, you'll have to delete your 2TB partition. This will allow you to select the GPT option (it's greyed out if you have formatted partitions) and will create a single unformatted partition. Once it's done reconfiguring you can create a new simple volume using the wizard. I would leave everything default except the name of the drive and do a quick format. It should format in a minute or less depending on the speed of your computer. Now you have one big 3TB partition again! Woohoo!
Hope that helps.