Prior to adding a destination to JetBackup, we advise securing the backup directory, so that only the allowed users can access that folder.
mkdir [-p] /mnt/backup_vol/jetbackup
Then we advise securing that destination so only one user can access it's content (except root)
useradd <jetbackup>
chown jetbackup:jetbackup /mnt/backup_vol/jetbackup
chmod 700 /mnt/backup_vol/jetbackup
Right now only two users that can access this directory are root and jetbackup.
Please search online, how to secure a backup folder for your particular file system (EXT, NTFS, FAT, etc..) on your system (Windows, Linux, macOS, BSD, etc..)
For more information, and a good start point please visit https://en.wikipedia.org/wiki/File_system_permissions
Now, when adding that destination in JetBackup GUI, the path field supplied should be the directory you have just created and secured: ie /mnt/backup_vol/jetbackup