Problem backing up on a mapped drive using Backup and Restore Cent

I am trying to set a a backup using Vista's "Backup and Restore Center" on to
a mapped Iomega networked drive. In the wizard, I select the "Back up files
Option", select the "On a network" option, and select the
\\iomega\netHDD\Backup folder I created. I'm asked to enter a Username and
Password. No usernames have been configured on the share (and I can browse
the mapped drive (N:\) freely). Leaving the fields blank, throws a "you must
provide a user name and password to access the shared network folder"
warning.

What is the problem and how can it be resolved? Thanks