Quoted from BOL, 'Backup Devices, Overview':
"If the backup is to be performed over the network to a disk on a remote computer, use the universal naming convention (UNC) name in the form \\Servername\Sharename\Path\File to specify the location of the file. As with writing files to the local hard disk, the appropriate permissions needed to read or write to the file on the remote disk must be granted to the user account used by SQL Server."
Jeff Mason
Custom Apps, Inc.
www.custom-apps.com