Windows Network Backup Help

[Christian]

I have PC upstairs running Windows 8 Pro, and a machine downstairs running Windows 7 Pro. Now I have a 3TB external HDD hooked up to the 7 downstairs, and a 1TB upstairs- in the hope of backing up each computer to the opposite one, so the backups are not in the same rooms as the computers.

I tried using Windows Backup to do it, and selecting Network backup. However I am then prompted to enter a username and password for the backup destination. I have tried the passwords to each PC with no luck, and basically everything else I could think of. Has anyone done this before? Or does anyone know of a detailed guide somewhere. I'm open to exploring third party backup programs too, but would probably prefer Windows Backup simply for the vast compatibility.

Thanks in advance for hep :)