Boot Camp partition format failure

I have been trying to install Windows 7 Professional on my late 2013 21.5" iMac running Mavericks.

The process works smoothly till I am asked to identify the partiton in which Windows is to be installed.

At this juncture I select the Bootcamp partition (as shown in the manual) and then select the Format option as the manual says I should.

At this point I get an error stating that the partition could not be formatted.

What should I do? All software is up to date and there are no updates pending. I am trying to install Windows 7 using an external Apple Superdrive.