On a Windows 2008 server, during a Deployment Server 6.9 SP6 install or upgrade on a Windows 2008 (or any server without .NET 4.0 installed), the system reboots interrupting the install.
Windows ADK (used to create WinPE 4.x) requires .NET 4.0. Windows 2008 and prior versions did not have .NET 4.0 installed, so during the Windows ADK install (which can be included as a part of the Deployment Server install) .NET 4.0 will be installed. The .NET 4.0 install includes a required reboot, which unfortunately happens even during the Deployment Server install.
The prerequisites of DS 6.9 SP6 states that .NET 4.0 needs to be installed before installing SP6. If this prerequisite is not met, the server will reboot during the install, however this means .NET 4.0 should be installed. Verify .NET 4.0 is installed, and start the DS 6.9 SP6 install again.