When tried to backup a mapped network drive with correct credential, It always failed with status code 71 (none of the files in the file list exist) even Netbackup client service has been set with required privilege.
Instead of mapping the network path via windows into a drive letter, using UNC path could resolve the issue. (i.e. \\hostname\path\)
Backup network drive option is not neccessarily set in policy in this case.