Backup from Server to Workstation External Drive

We have a Windows Server 2003 Domain server with a number of computers that are part of the domain.

My Windows 7 Pro computer is part of the domain.

I would like to do a complete server backup to an external USB 1TB hard drive that is plugged into my Win7 computer.

I mapped the drive as Backup (Z) on my Win7 machine.

I set it for SHARING and I set it for my username and Administrator to have Full Control.

When I go to configure backup on the Win2K3 server, I am able to browse to that external drive.

However, when I try to select it as the backup drive, it comes up first saying to make sure there are correct permissions for the Domain Admin (which I am), and then it pops up saying that it "unable to create the folder. Ensure that the disk has sufficient space and is not-read-only."

I'm not sure what to do at this point.

Any ideas?

(permissions permissions!)