Why can't I select "e" drive (external hard) to back up my files?

this seems to be a simple thing, but I can’t figure it out.  I have the My Book Home edition …

Trying to “create back up plan” for the first time.  I’m selecting “hard drive” as my destination with intentions of backing files up to the EXTERNAL WD Book that is plugged into the computer.  When selecting “click here to choose a back up folder” it only offers me the “c” drive to place my files.  Obviously that is not where I want my files to go.

am i missing the obvious??

hope someone can shed some light. 

Thx in advance - Lisa

See if this link helps you.