Installing SQL Server on not the default path
From BOL:

During installation of SQL Server 2012, Windows Installer creates temporary files on the system drive. Before you run Setup to install or upgrade SQL Server, verify that you have at least 6.0 GB of available disk space on the system drive for these files. This requirement applies even if you install SQL Server components to a non-default drive.

http://msdn.microsoft.com/en-us/library/ms143506(v=sql.110).aspx

You still need 6GB free on the system drive, regardless of where you are installing it.  Although, depending on your options for install, that may be smaller, as in your case.