When you create in Contributor Administration a new application in SQL server, You can only provide a data directory but not a log directory. Therefore the data and log files are created in the same directory. You can change this by editing the file epEAdminSQL7Resources.xml located in <installation directory>\c8\bin.
The SQL server log file directory is F:\SQL\Logs
In line 6 you find the following line for the log file location
LOG ON ( NAME = [%1_log], FILENAME = N'%2%1_log.ldf', SIZE = 10MB, FILEGROWTH = 10% )
change this into
LOG ON ( NAME = [%1_log], FILENAME = N'F:\SQL\Logs\%1_log.ldf', SIZE = 10MB, FILEGROWTH = 10% )
Save file and restart Cognos services.
Tip: you can change here also the initial size of the database and the file growth behavior