So far so good. But when he tried to apply SP1 for SQL Server 2008 R2 the installation failed, on both cluster nodes.
After some searching my colleague found the cause and the solution as well:
Setup of SP copies and unpacks all the files to the disk with the most free space available. In many cases it’s a a shared cluster disk. When the setup of SP is processing the SQL server, it takes SQL Server and the disks offline which results in a failed setup…
Extract the SP file on a disk which isn’t a shared cluster disk and run the setup from there. How to extract such a file? Run this command: SQLServer2008R2SP1-KB2528583-x64-ENU.exe /X. A dialog box will appear now prompting you to specify the path to where the extracted files must be saved:
When done, install the SP from that folder by running Setup.exe with elevated permissions.