The meaning of the codes on Failed BCK

I got this msg during the backup operations! - someone let me know why it is happening and where I'm doing wrong while my user that is connected to he SQL Server is in sysAdmin role and it is windows user in the Administrators Group. Normal BCK with SSMS are performing with no problems. I have permission to Read'n'Write everything in my Server.

Any way to avoid this failed operation!?



MyServer : Backup - Failed
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Performing full backups - Failed
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This operation failed with errors.

Backing up MyDB (full database) to:
MyPath:\FULL_(local)_MyDB_20130117_212623.sqb
Error 880: BACKUP DATABASE permission denied in database: (MyDB)
SQL error 916: The server principal "NT AUTHORITY\SYSTEM" is not able to access the database "MyDB" under the current security context.
SQL Backup exit code: 880
SQL error code: 916



Updating SQL Server information - Successful
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This operation completed successfully.