While upgrading Microsoft SQL Server 2014 SP2 to CU11 I got this error:
Use the following information to resolve the error, and then try the setup process again.
Feature failure reason:
An error occurred during the setup process of the feature.
§ Error installing SQL Server Database Engine Services Instance Features
The Database Engine system data directory in the registry is not valid.
Error code: 0x851A0042
Visit http://go.microsoft.com/fwlink?LinkId=20476&ProdName=Microsoft+SQL+Server&EvtSrc=setup.rll&EvtID=50000&ProdVer=12.0.5579.0&EvtType=0xD8FB5EBA%400x97A656BB%401306%4066&EvtType=0xD8FB5EBA%400x97A656BB%401306%4066 to get help on troubleshooting.
The part where it says „The Database Engine system data directory in the registry is not valid.“ pointed me in the right direction. Some time ago I moved the system data files to a new partition. While having a look at the registry I found that a path configured there was not valid:
The path for SQLDataRoot wasn’t valid any more. After changing it to the right path the upgrade finished without any issues.