When you have a stand-alone drive, you are responsible for loading the correct tape.
If the tape tape has passed it's OPP, then ANY job can write to it. If it has not passed the OPP it cannot be overwritten at all (unless, of course, you move it to the scratch set)
If the loaded tape tape is not in the same media set, you cannot append, even if the APP has not been passed.
The option to use Overwriteable media in the target media set before scratch media does no good, since BackupExec can only see the one tape volume.