SQL Server 2008 R2 Cluster Service pack 3 fail to failover node
I found the issue after a long time. It was due to 1 million+ files in the \MSSQL\log folder.

After setting up a job that cleared that folder. The failover after SP install worked fine.

The solution was confirmed on both this 2008R2 cluster, and the 2012 cluster we had the same problem on.