Backing up to network external HD

3 machines.
Vista wired connection and 2 XP wireless.
The external HD is connected to one of the XP machines.
Network printer connected to the same XP machine works fine.

When I try to select the external HD as the backup location I am asked for a
username and password. The drive was never password protected.

From any machine I can edit files and copy and paste to the drive without
having to enter credentials.

Any idea why I'm asked for credentials?
Please let me know if you need more info.
Thanks.