Time Machine

I have been trying , for a while to get time machine to work and I keep getting the following error message::

Time Machine Error

The backup disk image could not be created

I have gone and made all access permissions enabled for WD_backup and everyone else public and the NFS to full access.

What have I missed?

Is there a way to disable having to use passwords to the network drive?

Should I hook the drive up directly to my computer and reformat the drive?