Chace Zhang (MSFT)
Microsoft CSS Online Newsgroup Support
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Based on my research, if it's configured through SBS Backup, when it's time for the backup begin, the system account launches the Back Up Small Business task from the Task Scheduler. The task in turn executes the Backup Runner to begin the backup process. When Backup Runner starts, it changes the password of the SBS Backup User to meet the strong password requirements. Once the new password is set, the account is enabled (which is disabled by default), and Backup Runner assumes the credentials of this account to begin the actual backup job using the NTBackup.exe. Once the backup is completed and backup results are reported, for security reasons, Backup Runner will disable the SBS Backup User account. The account remains disabled until the next time Back Up Small Business Server task starts.
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Source: http://forums.msrportal.com/archive/index.php?t-7956.html