USMT 4.0 will run on both x86 and x64 platforms. When used on a Windows PE boot, you must have the correct platform of Windows PE for the architecture of the client you are booting.
Reference: http://technet.microsoft.com/en-us/library/ee126219(WS.10).aspx
Reference: http://technet.microsoft.com/en-us/library/ee126219(WS.10).aspx
Comments