Browse there manually using Windows Explorer and then cut&paste the path and backup name into the bottom text field of the dialog box that's showing you your folders. You cannot map a network drive in your windows session and have SQL Server use it as its running under a service account. I haven't tried but you could set SQL Server to log on using a specific user account and have that account have a mapped drive persistently mapped? Using a full UNC of the form \\computername\share\folders\file.bak is always going to be more reliable than a mapped drive letter.