I would like to write the results of a Buffer operation on Feature Class A to an existing Feature Class B where both A and B are on SQL Server. Both A and B are also published as Feature Layers to ArcGIS Server. When I try to build a Model to do this I keep getting errors to do with the fact that the Feature Class B already exists and/or there is a conflict with already established locks on the database.
The Buffer tool creates a new feature class to which to output its features, so that might be the reason you're getting the message about Feature Class B already existing? Instead, output the results of your Buffer operation to an intermediate feature class, then use the Append tool (http://resources.arcgis.com/en/help/main/10.1/index.html#//001700000050000000) to append these results to Feature Class B. Check then whether you're still getting messages about locks, and address that separately if you are.