To install TMG, then SP1 then RU1 for any reason, is a long way drive if you follow the normal process!

I found the below way to install the slipstreamed source of TMG with SP1 in one DVD. Follow the steps as shown below to end up with the upgraded copy

  1. Run the following from an elevated command prompt:

ServerManagerCmd.exe -inputpath ^<DVD_path^>\FPC\PreRequisiteInstallerFiles\WinRolesInstallSA_Win7.xml -logPath C:\Windows\TEMP\TMG-Prerequisites.log

  1. Prepare a Forefront TMG SP1 slipstream DVD with the following steps:
    1. Copy the Forefront TMG DVD and the Forefront TMG SP1 MSP file to a local drive on the target computer. For the purposes of this example, let’s assume this is c:\temp\TMG.
    2. At a command prompt, type the following command and press ENTER.
      msiexec /a c:\temp\TMG\FPC\MS_FPC_SERVER.msi /p TMG-KB981324-amd64-ENU.msp /qb /L*v c:\tmg\log.txt
      When the operation completes, you will have a full installation of Forefront TMG already upgraded to Service Pack 1.

 

Enjoy.