The Windows 7 update "System Update Readiness Tool for Windows 7 (KB947821) [May 2014]" will appear hung or stuck during the update process. It will get to about 80 to 90% and just the hard drive activity is all that will indicate that it is still working on the update process. Many have noted it to take several hours. In our own install of this for a client it took almost 6 hours to complete on a system that failed the SFC scan test.
If you are installing the System Update Readiness Tool for Windows 7 (KB947821) besure to allow for a long period of time to install. Do not stop this install or you may corrupt the system to a point where it will not boot and you will be required to reload Windows from scratch.