How to find existing instances "instance root" (MS SQL)?
Found it by comparing several articles and looking at existing file structure and registry under:   HKLM\software\Microsoft\Microsoft SQL Server\MSSQL10.INSTANCEID\Setup
There are a few "path" and "dir" references, most of them quite long. The common denominator seems to be the SqlProgramDir key, for all the existing instances. In our case the DBA decided to put them all under "D:\SQL\Data\"