I had the same problem and this solution worked for me:
http://community.wd.com/t5/My-Book-Live-Duo/Share-Access/m-p/533175/highlight/true#M3447
Apparently, if Public is mapped first or already mapped, Windows won’t allow any private drives to map. I disconnected Public (I did this by right-clicking and hitting Disconnect), and then I could map all privates, and then I could map the Public. I did not have to go through all the CMD stuff as noted above.
