If you get an error applying Service Pack 2 to SQL 2008 R2:
The patch installer has failed to update the following instance: MSSQLSERVER. To determine the reason for failure, review the log files.
And then if you go to the log file it mentions
Product: SQL Server 2008 R2 SP2 BI Development Studio – Update ‘Service Pack 2 for SQL Server Business Intelligence Development Studio 2008 (64-bit) (KB2630458)’ could not be installed. Error code 1603.
The problem occurs if you have deleted any files with the .log extension, or used the Microsoft Disk Cleanup wizard. To fix it, go to Start > All Programs > Microsoft SQL Server 2008 R2 > Configuration Tools > SQL Server Installation Center and run the wizard. When prompted to choose which instance, select the option titled “<<Repair shared features only>>” and click Next, Next, Repair. Then re-run the Service Pack and everything should install correctly.