SQL Server Backup transaction error
You appear to have corruption in your transaction log file. This needs to e resolved before you continue with you mirroring configuration.


  Where is the error log?


Using SSMS the error log can be found in the management folder.

I would check the log for more information and run a full DBCC CHECKDB to ensure the database is OK.

Paul Randall is an expert in data corruption in SQL Server and he mentions a fix to your problem in this article. I would read this through to get a good understanding of the situation you are in. In brief he says that the situation may be solved by switching the database to SIMPLE recovery mode, back to FULL recovery mode and then performing a FULL backup. The full backup restarts the log chain.

I would also inspect the hard drives that the transaction log file is running on as they may have caused the corruption if they are faulty.