I first described the /UM switch in tip 2265 ยป How do I perform an NTBackup in Windows 2000, without having to manually manage the media?

Microsoft Knowledge Base Article 295159 contains the following description:

When Windows 2000 Backup (Ntbackup.exe) is scheduled to run at a later time and you choose to overwrite the existing tape media by using the /um switch, the backup never completes. When you look at Scheduled Tasks, the jobs status reports it is...