Whilst setting up a new WSUS server on Windows Server 2012 R2, when entering the content path such as W:\ and clicked Run I received the following error message:

Log file is located at C:\Users\administrator.corp\AppData\Local\Temp\tmpE3AA.tmp
Post install is starting
Fatal Error: Illegal characters in path.

Although I had done it before it turns out that you cannot specify the root partition, you need to specify a directory such as W:\WSUS. Changing the path resolved the issue and I was able to continue installing.